CHAPTER III.

PHENOMENA OF SIGHT, AND ART PRINCIPLES DEDUCED THEREFROM.

[Sidenote: Introduction.]

Having thus demonstrated that the best artists have always tried to interpret nature, and express by their art an impression of nature as nearly as possible similar to that made on the retina of the human eye, it will be well to inquire on scientific grounds what the normal human eye really does see.

[Sidenote: The argument.]

Our contention is that a picture should be a translation of a scene as seen by the normal human eye. That the impression will vary with individuals, there is no doubt, for the artist will see subtleties never dreamed of by the commonplace or uneducated eye, and his aim will, of course, be to portray those subtleties in his picture, and hence one source of individuality in a work, another being in the way in which it is done. Our task now shall be to examine into the physical, physiological and psychological properties of sight, and to arrive at a conclusion, in so far as science allows us, as to how the normal eye does see things. The student will do well to read Chapter II. of Book III. of Dr. Michael Foster’s “Text Book of Physiology,” as well as the matter on the eye in Ganot’s Physics, before going any further in this chapter, for we do not wish to go over ground which has been occupied previously, our aim being to give a view from the artistic standpoint of the physical, physiological, and psychological properties of eyesight. We will, then, proceed to consider how well we see external nature, that is, within what limits, for we never see her exactly as she is, as we shall show.

[Sidenote: Optic nerves.]

To begin with, then, the retinal nerves are strictly reserved to respond to the vibrations of ether—called light. If the student has ever had a blow on his eye, he has probably _seen_ “stars,” because every stimulus to this pair of nerves makes us see things, and not feel them. Now each sense has certain limits between which it can detect subtle vibrations, but beyond which all is blank. The more refined the organization of the person, the greater will be the number of vibrations he can distinguish. Thus 399,000,000,000 vibrations in a second produce in us the sensation of light, above this the vibrations appear as spectral colours until the number 831,000,000,000,000 is reached; to an increase in the number of vibrations above that number the optic nerve does not respond. Now the eye is an optical apparatus fixed between the brain and the ether, not that we may perceive light, for we could do that without the eye, but that we may distinguish objects. The glyptic and pictorial arts are founded entirely on the sense of sight as music is founded on the sense of hearing. In the pictorial arts, then, we must clearly distinguish between the physical, physiological, and psychological properties of sight.

[Sidenote: Le Conte’s division.]

Le Conte divides the scientific, i.e. physical and physiological data, into: A. Light; B. Direction of Light; C. Intensity; D. Colour; and the psychological data into Binocular vision, size, solidity, and depth. Following up Le Conte’s scheme, let us begin, then, to discuss briefly the scientific data, that is, considering the apparatus purely from the standpoint of physics and physiology.

A. LIGHT.

[Sidenote: Light.]

I. Physical characters of the eye as an optical instrument.

If a ray of light passes through a small hole into a darkened room (pin-hole camera), an image is formed of the object or objects without. The condition of a good definition of the image is that “all the rays from each point on the object must be carried to its own point on the image.” If this hole be enlarged, this condition is impossible, and the light spreads over certain areas called diffusion areas or diffusion circles. In other words, widely divergent rays and contiguous rays become mixed. To admit more light a lens is used in the eye, and by the photographer, for although it is possible (by pin-hole camera) to take pictures without a lens, the light so admitted is necessarily so limited that the exposure needed is too long. The lens, however, helps us by admitting more light, and at the same time giving better definition, but it also introduces many disadvantages and sources of error. Now a _theoretically_ perfect physical image has been described by physicists as being both bright and sharp in definition, but the theoretically perfect image does not exist; for, apart from other considerations, the lens which we use to get microscopic sharpness, cuts off light, and the sharper the image is rendered by stops, the less brightness do we get. Thus we see the lens introduces scores of errors as well as desirable qualities.

In the human and photographic lenses the chief faults are:—

[Sidenote: Dispersion.]

Dispersion. All refraction or bending of light by a lens is accompanied by dispersion. This error is corrected in opticians' lenses to a great extent. In the human eye, however, this fault is in some degree present, as can be proved by looking at a lighted street lamp through a violet glass, when a red flame will be seen surrounded by a bluish-violet halo. What, then, is the effect of dispersion on our theoretically perfect image? It is slight blurring of the sharpness of outline, since the size and position of the optical images thrown by the differently bent rays is not the same.

[Sidenote: Spherical aberration.]

A lens having a spherical surface bends the rays so that they do not all come to a focus at the same point. What is the effect of this on our theoretically perfect image? Again it is slight blurring of the sharpness of outline. It is said the spherical aberration in a perfectly corrected optician’s lens _is less than that in the lens of the human eye_. This must be remembered in connection with our later remarks. In the lower animals, spherical aberration is nearly absent. Their vision therefore is more periscopic, and therefore more like that of an optician’s lens.

[Sidenote: Astigmatism.]

This defect can be avoided in the optician’s lens, but it exists in, and is a serious fault of, the human eye.

Helmholtz considers the amount of spherical aberration unimportant as compared with this defect. Astigmatism is the result of imperfect symmetrical curvature of the cornea and of imperfect centering of the cornea and lens. This defect is found in most human eyes.

Astigmatism prevents the eye seeing vertical and horizontal lines at the same distance perfectly clearly at once. The defect in centering also causes irregular radiation, so that, as Helmholtz says, “The images of an illuminated point as the human eye brings them to focus, are inaccurate.” What is the effect of those defects on the “perfect image”? Dimness of outline and detail in the textures of objects seen.

[Sidenote: Turbidity.]

The optician’s lens is made of pure glass, the media of the human eye are not clear, but slightly turbid, so that Helmholtz says, “The obscurity of dark objects when seen near very bright ones depends essentially on this defect. This defect is most apparent in the blue and violet rays of the solar spectrum; for then comes in the phenomena of fluorescence to increase it.” [Sidenote: Fluorescence.] By fluorescence is meant the property which certain minutely divided substances possess of becoming faintly luminous, so long as they receive violet and blue light. The bottles filled with solution containing quinine, which look blue in the chemists' windows, owe their colour to this fact, as also does the blueness of “London” milk. These defects, combined with entoptic impurities which are constantly floating about in the humours, all help to detract from the brightness and sharpness of the “perfect image.”

[Sidenote: Blind spot.]

This is a portion of the retinal field with no cones or rods, and therefore insensitive to light. This causes a gap in the field of vision. “This blind spot is so large that it might prevent our seeing eleven full moons if placed side by side, or a man’s face at a distance of only six or seven feet,” says Helmholtz. In addition to this, there are lesser gaps in the retinal field, due to the cutting off of light by the shadows thrown by the blood vessels. Any one who has examined the retinal field with an ophthalmoscope knows what this means.

[Sidenote: Macula lutea.]

In addition to this the _macula lutea_ is less sensitive to weak light than other parts of the retina. The effect of all these imperfections is to blur and dull the perfect image. The serious defects due to the blind spot are not noticed, according to Helmholtz, because “we are continually moving the eye, and also that the imperfections _almost always affect those parts of the field to which we are not at the moment directing our attention_.” The italics are ours. Here, then, is another great difference between the eye and the optician’s lens.

[Sidenote: Focussing.]

The focus of the eye in a passive state is adjusted to the most distant objects. It focusses for nearer objects by contracting the ciliary muscle which pulls tight the zonule of Zinn and so curves the crystalline lens. It can focus thus up to within five inches of itself, but the changes of focus are almost imperceptible to the eye beyond twenty feet. Now a theoretically perfect eye might form perfect images of objects at infinite distances when there were no intervening objects. But as has already been shown, the eye is very imperfect, and its images are not therefore perfect, and it could not form theoretically perfect images, even if the atmosphere were pure ether and nothing else, for there are other facts in nature which prevent this; thus we cannot see a sharp image of the sun with the naked eye on account of its dazzling brightness.

[Sidenote: Fovea centralis.]

This central spot is a most important factor in the study of sight and art. For though the field of vision of the two eyes is more than 180° laterally, and 120° vertically, yet the field of distinct vision is but a fraction of this field, as we can all prove for ourselves. Now the field of distinct vision depends on the central spots for the reason that the central spot differs anatomically from the rest of the retina by the absence of certain layers which we need not specify here. The absence of these layers exposes the retinal bacillary layer to the direct action of light. Helmholtz says “all other parts of the retinal image beyond that which falls on the central spot are imperfectly seen,” so that the image which we receive by the eye is like a picture minutely and elaborately finished in the centre, but only roughly sketched in at the borders. But although at each instant we only see a very small part of the field of vision accurately, “_we see this in combination with what surrounds it, and enough of this outer and larger part of the field, to notice any striking object, and particularly any change that takes place in it_.” If the objects are small, they cannot be discerned with the rest of the retina, thus, to see a lark in the sky, Helmholtz says it must be focussed on the central spot. Finally he says, [Sidenote: Direct and indirect vision.] “To _look_ at anything means to place the eye in such a position that the image of the object falls on the small region of perfectly clear vision. This we may call _direct_ vision, applying the term _indirect_ to that exercised with the lateral parts of the retina, indeed with all except the central spot.” Again, he says, “Whatever we want to see we look at and see it accurately; what we do not look at, we do not as a rule care for at the moment, and so do not notice how imperfectly we see it.” Now all this is most important in connection with art, as we shall show later, we must beg the student therefore to hold it fast.

It will be seen from all this that a perfect periscopic image is never seen by the eye of man, though in some of the lower animals the matter may be different.

B. DIRECTION OF LIGHT.

[Sidenote: Law of projection.]

Le Conte says, “The retinal image impresses the retina in a definite way; this impression is then conveyed by the optic nerve to the brain, and determines changes there, definite in proportion to the distinctness of the retinal image, and then the brain or the mind refers or projects this impression outward into space as an _external image, the sign and facsimile of an object_ which produces it.” Not only does this hold good of external images, but in certain diseases retinal impressions arising from within are projected outwards, thus ghosts are _seen_.

[Sidenote: Corresponding points, &c.]

“From Müller’s law,” Le Conte further says, “it is evident that each point—every rod or cone—in the retina has its invariable correspondent in the visual field, and _vice versâ_.”

[Sidenote: Law of visible direction.]

Le Conte’s law of visible direction states that, “Where the rays from any radiant strike the retina the impression is referred back along the ray line (the central ray of the pencil) into space, and therefore to its proper place.”

From these laws we understand why we see things in the relative positions which they occupy in space.

All the previous remarks are applicable to monocular vision.

C. INTENSITY.

[Sidenote: Intensity.]

A quotation from Helmholtz will best illustrate this point. He says, “If the artist is to imitate exactly the impression which the object produces on our eye, he ought to be able to dispose of brightness and darkness equal to that which nature offers. But of this there can be no idea. Let me give a case in point. Let there be in a picture-gallery a desert scene, in which a procession of Bedouins, shrouded in white, and of dark negroes, marches under the burning sunshine; close to it a bluish moonlight scene, where the moon is reflected in the water, and groups of trees, and human forms, are seen to be faintly indicated in the darkness. You know from experience that both pictures, if they are well done, can produce with surprising vividness the representation of their objects; and yet in both pictures the brightest parts are produced with the same white lead, which is but slightly altered by admixtures; while the darkest parts are produced with black. Both being hung on the same wall, share the same light, and the brightest as well as the darkest parts of the two scarcely differ as concerns the degree of their brightness.

How is it, however, with the actual degrees of brightness represented. The relation between the lightness of the sun’s light, and that of the moon, was measured by Wollaston, who compared their intensities with that of the light of candles of the same material. He thus found that the luminosity of the sun is 800,000 times that of the brightest light of a full moon.

An opaque body, which is lighted from any source whatever, can, even in the most favourable case, only emit as much light as falls upon it. Yet, from Lambert’s observations, even the whitest bodies only reflect about two-fifths of the incident light. The sun’s rays, which proceed parallel from the sun, whose diameter is 85,000 miles, when they reach us, are distributed uniformly over a sphere of 195 millions of miles in diameter. Its density and illuminating power is here only one-forty-thousandth of that with which it left the sun’s surface; and Lambert’s number leads to the conclusion that even the brightest white surface on which the sun’s rays fall vertically, has only the one-hundred-thousandth part of the brightness of the sun’s disk. The moon, however, is a grey body, whose mean brightness is only about one-fifth that of the purest white.

And when the moon irradiates a body of the purest white on the earth, its brightness is only the hundred-thousandth part of the brightness of the moon itself; hence the sun’s disk is 80,000 million times brighter than a white which is irradiated by the full moon.

Now, pictures which hang in a room are not lighted by the direct light of the sun, but by that which is reflected from the sky and clouds. I do not know of any direct measurements of the ordinary brightness of the light in a picture-gallery; but estimates may be made from known data. With strong upper light, and bright light from the clouds, the purest white on a picture has probably 1-20th of the brightness of white directly lighted by the sun; it will generally be only 1-40th, or even less.

Hence the painter of the desert, even if he gives up the representation of the sun’s disk, which is always very imperfect, will have to represent the glaringly lighted garments of his Bedouins with a white which, in the most favourable case, shows only the 1-20th part of the brightness which corresponds to actual fact. If he could bring it, with its lighting unchanged, into the desert near the white there, it would seem like a dark grey. I found, in fact, by an experiment, that lamp-black, lighted by the sun, is not less than half as bright as shaded white in the brighter part of a room.

On the picture of the moon the same white which has been used for depicting the Bedouins' garments must be used for representing the moon’s disk, and its reflection in the water; although the real moon has only one-fifth of this brightness, and its reflection in water still less. Hence white garments in moonlight, or marble surfaces, even when the artist gives them a grey shade, will always be ten to twenty times as bright in his picture as they are in reality.

On the other hand, the darkest black which the artist could apply would be scarcely sufficient to represent the real illumination of a white object on which the moon shone. For even the deadest black coatings of lamp-black and black velvet, when powerfully lighted, appear grey, as we often enough know to our cost, when we wish to shut off superfluous light. I investigated a coating of lamp-black, and found its brightness to be about one-hundredth that of white paper. The brightest colours of a painter are only about one hundred times as bright as his darkest shades.

The statements I have made may appear exaggerated. But they depend upon measurements, and you can control them by well-known observations. According to Wollaston, the light of the full moon is equal to that of a candle burning at a distance of twelve feet. Now, assume that you suddenly go from a room in daylight to a vault perfectly dark, with the exception of the light of a single candle. You would at first think you were in absolute darkness, and at most you would only recognize the candle itself. In any case, you would not recognize the slightest trace of any objects at a distance of thirteen feet from the candle. These, however, are the objects whose illumination is the same as that which the moonlight gives. You would only become accustomed to the darkness after some time, and you would then find your way about without difficulty.

If now, you return to the daylight, which before was perfectly comfortable, it will appear so dazzling that you will, perhaps, have to close your eyes, and only be able to gaze round with a painful glare. You see thus that we are concerned here not with minute, but with colossal, differences. How now is it possible that, under such circumstances, we can imagine there is any similarity between the picture and reality?

Our discussion of what we did not see at first, but could afterwards see in the vault, points to the most important element in the solution; it is the varying extent to which our senses are deadened by light; a process to which we can attach the same name, fatigue, as that for the corresponding one in the muscle. Any activity of our nervous system diminishes its power for the time being. The muscle is tired by work, the brain is tired by thinking, and by mental operations; the eye is tired by light, and the more so the more powerful the light. Fatigue makes it dull and insensitive to new impressions, so that it appreciates strong ones only moderately, and weak ones not at all.

But now you see how different is the aim of the artist when these circumstances are taken into account. The eye of the traveller in the desert, who is looking at the caravan, has been dulled to the last degree by the dazzling sunshine; while that of the wanderer by moonlight has been raised to the extreme of sensitiveness. The condition of one who is looking at a picture differs from both the above cases, by possessing a certain mean degree of sensitiveness. Accordingly, the painter must endeavour to produce by his colours, on the moderately sensitive eye of the spectator, the same impression as that which the desert, on the one hand, produces on the deadened, and the moonlight, on the other hand, creates on the untired eye of its observer. Hence, along with the actual luminous phenomena of the outer world, the different physiological conditions of the eye play a most important part in the work of the artist. What he has to give is not a mere transcript of the object, but a translation of his impression into another scale of sensitiveness, which belongs to a different degree of impressibility of the observing eye, in which the organ speaks a very different dialect in responding to the impressions of the outer world.

[Sidenote: Fechner’s law.]

In order to understand to what conclusions this leads, I must first explain the law which Fechner discovered for the scale of sensitiveness of the eye, which is a particular case of the more general _psycho-physical law_ of the relations of the various sensuous impressions to the irritations which produce them. This law may be expressed as follows:—_Within very wide limits of brightness, differences in the strength of light are equally distinct, or appear equal in sensation, if they form an equal fraction of the total quantity of light compared._

Thus, for instance, differences in intensity of one-hundredth of the total amount can be recognized without great trouble, with very different strengths of light, without exhibiting material differences in the certainty and facility of the estimate, whether the brightest daylight, or the light of a good candle be used.”

Herein, then, are contained the limits with which we can work, and the physiological reasons why we can render a fairly true impression of a scene in nature.

The only constant factor, then, is the _ratio of luminous intensities_,—that is, the picture must be as true as possible in relative tones or values. Obviously a picture of bright sunlight should look brighter in a moderately lighted room than the surrounding room, that is, its first impression on the observer should be as if he were looking at a landscape beyond the walls, through the frame.

From these remarks it will be seen how utterly impossible it is to render truly a bright sunlight scene, for if the values be true, starting from the top of the scale, the highest light, when you get to the middle tints, they are too black already, and the picture is out of tone and false. Obviously the right way is to start from the lower end of the scale, the _darks_, and get them as true as possible, and let the lights take care of themselves; but more of this anon.

D. COLOUR.

[Sidenote: Colour.]

As photographers, the matter of colour exercises us but indirectly, still the subject should be understood, on account of its bearing on painting. “Colour perception” says Le Conte, “is a single perception, and irresolvable with any other. It must, therefore, have its basis in retinal structure.”

Helmholtz divides the vibrations of ether known as light into three degrees. He says the longest and shortest rays do not essentially differ in any other physical property, except that we distinguish them from the intermediate waves.” Thus the ear can receive at once many waves of sound or notes, and they remain distinct, but notes of colour do not keep distinct in the same way, “so that the eye is capable of recognizing few differences in quality of light,” says Helmholtz, and can only perceive the elementary sensation of colour by artificial preparation. He also says, the only bond between the objective and subjective phenomena of colour may be stated as a law thus, “Similar light produces under like conditions a like sensation of colour. Light, which under like conditions, excites unlike sensations of colour is dissimilar;” what we want in art, then, is the _appearance_ of the phenomena. The illumination of the sun’s rays cannot be weakened without at the same time weakening their heating and chemical action; this is a point to be remembered in exposure.

Colour is, of course, excited by the length of the waves and their frequency, red being the longest and slowest, and they diminish in length and increase in frequency in the order of the spectrum through orange, yellow, green, blue, indigo, to the shortest waves, which produce the effect of violet, the whole combined forming white. Now Hering has shown that there are only four primary colour sensations, though he at one time included black and white, thus making six. The four are red, yellow, green, and blue, which are reduced by him to two complementary colours, red and green, and yellow and blue. In our present state of knowledge the Young-Helmholtz theory of three primary colour sensations for red, green, and blue seems preferable as a working hypothesis, though it seems incompatible with anatomical and physiological facts.

[Sidenote: Difference of colour.]

All objective differences between colours, according to Helmholtz, may be reduced to differences of tone, difference of fulness (saturation), and difference of brightness. These are the three colour constants.

By tone, or hue, he means in fact difference of colour as in the spectral colours. He here refers to the vibration on a tonic scale. Fulness or purity is greatest in the pure tints of the spectrum, and becomes less in proportion as they are mixed with white light. All compound colours are less full than the simple hues of the spectrum.

Brightness or luminosity is strength of light, or amount of illumination. It is measured by the total amount of light reflected to the eye.

In nature black and white must be included among the primary colours when _quality_ is spoken of, as light acts on black and white.

All differences of tone, therefore, are the result of combinations in different proportions of the four primary colours.

Among the defects of the eye in seeing colour, Helmholtz says, “All are red blind at the innermost portion of the field of vision, all red colours appear darker when viewed indirectly.”

The furthest limit of visible field is a narrow zone, in which all distribution of colour ceases, and there only remain differences of brightness. Probably those nervous fibres which convey impressions of green light are alone present in this part of the retina. The yellow spot makes all blue light appear somewhat darker in the centre of the field.

All these inequalities are known and more or less rectified by constant movement. As the eye becomes fatigued by bright light, so that it cannot at first answer to delicate stimulus, so it can become partially fatigued for certain colours.

Fatigue weakens the apparent illumination of the entire field of vision.

The colour of illumination of a picture, too, varies greatly by effect of local colour.

What is constant in the colour of an object is not the brightness and colour of the light which it reflects, _but the relation between the intensity of the different-coloured constituents of this light, on the one hand, and that of the corresponding constituents of the light which illuminates it on the other_. For example, white paper in full moonlight is darker than black satin in daylight, or a dark object with the sun shining on it reflects light of exactly the same colour, and perhaps the same brightness, as a white object in shadow. Grey in shadow looks like white.

Brightness of local colour diminishes with the illumination or as the fatigue of the retina is increased. In sunshine, local colours of moderate brightness approach the brightest, whereas in moonlight they approach the darkest. Pictures to be seen in daylight do not admit of difference of brightness between sun and moon. As colours increase in brightness, red and yellow become apparently stronger than blue. Painters make yellow tints predominate when representing landscape in full sunshine, while moonlight scenes are blued. Helmholtz says:—“Differences of colour which are actually before our eyes are more easily apprehended than those which we only keep in memory, and contrast between objects which are close to one another in the field of vision are more easily recognized than when they are at a distance. All this contributes to the effect. Indeed, there are a number of subordinate circumstances affecting the result which it would be very interesting to follow out in detail, for they throw great light upon the way in which we judge of local colour; but we must not pursue the inquiry further here. I will only remark that all these effects of contrast are not less interesting for the scientific painter than for the physiologist, since he must often exaggerate the natural phenomenon of contrast in order to produce the impression of greater varieties of light and greater fulness of colour than can be actually produced by artificial pigments.”

Again, when turbidity is composed of fine particles its appearance is blue, as the mists seen in autumn hanging round coverts, but it is whiter than the aërial blue because of the colour of the covert behind. When this turbidity is absent the colours are brighter, hence the fierce blue on bright sunshiny days with easterly winds. This matter of turbidity must not be forgotten in portrait work; it is this which helps to give relief, hence the absurdity of all photographers' devices, the object of which is to minimize this turbidity. In addition to these is the ever-changing effect of atmosphere on colour, that subtle medium with which the enchantress Nature produces ever-changing effects, and its chief effect on colour is to lower it in brightness. _Atmosphere greys all things_, hence on a misty day all the colours are greyed—we have, in fact, a “grey day.”

Another point which must not be forgotten is that with bright illumination bright objects become more like the brightest, and with feeble illumination dark objects become more like the darkest. This is a very important matter, for it means that in bright sunshine the lightest greys are lost in white, whilst in dull weather the darkest greys are lost in black, hence the falsity of having deep blacks in brightly-lighted landscapes, and as has been shown, these are untrue, and the result of ignorance and of faulty manipulation. As Helmholtz has it, “The difference of brightness and not absolute brightness; and that the differences in them in this latter respect can be shown without perceptible incongruity if only their graduations are imitated with expression.”

E. BINOCULAR VISION—PSYCHOLOGICAL DATA.

_Single Image._

[Sidenote: Binocular Vision.]

The remarks already made would apply equally well to man if he were a one-eyed animal, but we find there are other considerations to take into account since man is two-eyed. Now the phenomena of binocular vision cannot be treated of with such accuracy as the physical and physiological facts already discussed. In this subject we shall follow Le Conte. It is obvious there is a common binocular field of view for the two eyes. Now Dr. Le Conte shows us that we see all objects double, except under certain conditions. When we look directly at anything, then we see it clearly, but all things nearer to us than the object looked at and beyond it, are seen double, or blurred and indistinct. This is the case in life, as can be proved.

He goes on to tell us that we see things singly when the two images of that thing are projected outward to the same spot in space, and are therefore superimposed and coincide. Objects are seen single when their retinal images fall on corresponding points—that is, objects lying in a horizontal circle passing through the point of sight and the central spots are seen single. Now “all objects at the same or nearly the same distance, but a little to the right or left, or above or below, are also either seen single, or else the doubling, if any, is usually imperceptible.” This surface of single vision is called the _horopter_.

There are, then, two adjustments, the focal and the axial, the one an adjustment for distant vision, the other for single vision, and connected with these is the adjustment of the pupil, which contracts and expands, not only to light, but also to distance and nearness of the object. Therefore, three adjustments take place when we look at anything. Connected with these laws are the laws of direction and corresponding points. Thus we see our perfect image can only exist in one place at once, that all between the eye and the object and beyond the object is indistinct, and that the further off an object is the more luminous does it appear. Two objects, too, may be seen as one.

F. PERSPECTIVE.

_Depth, Size, and Solidity._

[Sidenote: Perspective.]

The next question is, “To what is due the appearance of solidity and depth?”

Depth, or relative distance, is judged of by a combination of four kinds of perspective.

1. _Focal or monocular perspective._—Objects at the point of sight are sharp, but all objects beyond or within this distance are dim. Distance is judged partly by the act of focussing the eye by acting, as we have said, on the lens. As this power only acts within twenty feet, it is evident that things can only be in focus in one plane.

2. _Mathematical Perspective._—Objects become smaller in appearance and nearer together as they recede. This is another aid to the judging of distance. The true rendering of this perspective in photography depends on the correct use of the lens, as will be explained.

3. _Aërial Perspective_ is the perspective due to the scattering of light by aërial turbidity, for the atmosphere always contains floating particles of matter. As the objects recede this curtain of turbidity becomes thicker and the distant objects grow dimmer and bluer. This is another aid to the judging of distance, but any one not accustomed to count on this effect may easily misjudge, as we have done before now to our cost in Switzerland, where a peak miles away has, at times, seemed to be in the next valley.

4. _Binocular Perspective_ is due to the convergence of the optic axes and formation of a single image. Le Conte says, “The perspective of depth or relative distance, whether in a single object or in a scene, is the result of the successive combinations of the different parts of the two dissimilar images of the object on the scene.” Binocular perspective, too, gathers together the imperfect retinal impressions when the eye sweeps over the field of view. This only acts within a few hundred yards.

Thus, then, in taking a photograph we must remember that theoretically speaking, up to twenty feet the picture can be made sharper all over than beyond that distance; for the eye has all these perspectives acting within that distance.

[Sidenote: Size.]

By size we estimate distance.

[Sidenote: Solidity.]

Solidity is judged by binocular vision and lighting.

When to all these difficulties are added those dependent on the subtleties of light reflected into shadow, and the thousand-and-one changes of colour due to the numerous shadows cast by objects in nature, we get a complexity which forces upon us how impossible it is for man to _copy_ nature. A “mere transcript of nature,” which is so glibly talked of, is, humanly speaking, an impossibility. No man ever painted a “mere transcript” of nature, or a truthful copy, any more than a man can make plants or animals in a laboratory; but he can, by a picture, give a truthful impression of nature.

On these data and within these limits, then, must we work, and here we append a few general principles deduced from these data, which must guide us in our work. We have followed them ourselves, and they form the scientific part of our creed of “Naturalistic Photography.” We have said little upon the drawing of photographic lenses, as that is discussed in another chapter; but of course Naturalistic Photography claims as of vital importance that lenses be used so as to give the drawing of objects as they are seen by the eye—in other words, as they would be drawn by a good draughtsman.

ART PRINCIPLES DEDUCTED FROM THE DATA CITED.

[Sidenote: Art Principles.]

We have shown why the human eye does not see nature exactly as she is, but sees instead a number of signs which represent nature, signs which the eye grows accustomed to, and which from habit we call nature herself. We shall now discuss the relation of pictorial art to nature, and shall show the fallacy of calling the most scientifically perfect images obtained with photographic lenses artistically true. They are not correct, as we have shown, and shall again show, but what is artistically true is really what we have all along advocated; that is that the photographer must so use his technique as to render a true impression of the scene. The great heresy of ‘sharpness’ has lived so long in photographic circles because firstly the art has been practised by scientists, and secondly by unphilosophical scientists, for all through the lens has been considered purely from the physical point of view, the far more important physiological and psychological standpoints being entirely ignored, so that but one-third of the truth has been hitherto stated.

[Sidenote: What a picture is.]

To begin with, it must be remembered that a picture is a representation on a plane surface of limited area of certain physical facts in the world around us, for abstract ideas cannot be expressed by painting. In all the works in the world the painter, if he has tried to express the unseen or the supernatural, has expressed the unnatural. If he paints a dragon, you find it is a distorted picture of some animal already existing; if he paints a deity, it is but a kind of man after all. No brain can conjure up and set down on paper a monster such as has never existed, or in which there are no parts homologous with some parts of a living or fossil creature. We defy any man to draw a devil, for example, that is totally unlike anything in existence. All so-called imaginative works fall then within the category of the real, for they are in certain parts real because they are all based on realities, even though they may be utterly false to the appearance of reality. By this we mean that an ideal dragon may be based on existing animals; his form may be a mixture of a Cobra, Saurian, and a reptile, as is often the case; so far it may be real, but then the way in which it is painted may be utterly false, for the natural effect of light and atmosphere on the dragon may and probably will be ignored, for there is no such animal to study from. The modern pre-Raphaelites are good examples of painters who painted in this way; they painted details, they imitated the local colour and texture of objects, but for all that their pictures are as false as false can be, for they neglected those subtleties of light and colour and atmosphere which pervade all nature, and which are as important as form. Children and savages make this same error, they imitate the local colour, not the true colour as modified by light, adjacent colour, and atmosphere. But what the most advanced thinkers of art in all ages have sought for is the rendering of the true impression of nature.

Proceed we now to discuss the component parts of this impression.

[Sidenote: Tone and Atmosphere.]

When we open our eyes in the morning the first thing we see is light, the result of those all-pervading vibrations of ether. The effects of light on all the objects of nature and on sight have been dealt with in the beginning of this chapter, it only remains, therefore, to deduce our limits from these facts. In the first place, from what has been said in that section it is evident we cannot compete with painting, for we are unable to pitch our pictures in so high a key as the painter does, and how limited is his scale has been shown, but by the aid of pigments he can go higher than we can. It has been shown, too, that it is impossible to have the values correct _throughout_ a picture, for that would make the picture too black and untrue in many parts. This fact shows how wrong are those photographers who maintain that every photograph should have a patch of pure white and a patch of pure black, and that all the lighting should be nicely gradated between these two extremes. This idea arose, no doubt, from comparing photography with other incomplete methods of translation, such as line-engraving.

The real point is that the darks of the picture shall be in true relation, and the high lights must take care of themselves. By this means a truer tone is obtained throughout. Now to have these tones in true relation it is of course implied that the local colours must be truly rendered, yellow must not come out black, or blue as white, therefore it is evident that colour-corrected plates are necessary. But such plates are useless when the quantity of silver in the film is little, for the subtleties of delicate tonality are lost, which are not compensated for by gain in local colour, and this is a point the makers of orthochromatic plates must take into consideration. It will be seen now why photographs on uncorrected plates (even when the greatest care and knowledge in using them is exercised) are not, as a rule, perfectly successful, and why the ordinary silver printing-paper is undesirable, for it exaggerates the darkness of the shadows, a fatal error. False tonality destroys the sense of atmosphere, in fact, for the true rendering of atmosphere, a photograph must be relatively true in tone; in other words the relative tones, in shadow and half shadow, must be true. If a picture is of a bright, sunlit subject, brilliancy is of course a necessary quality, and by brilliancy is _not_ meant that “sparkle” which so delights the craftsman. Of course the start of tone is naturally made from less deep shadows, when the picture is brightly lighted, for the black itself reflects light, and all the shadows are filled with reflected light. It will be seen, therefore, that it is of paramount importance that the shadows shall not be too black, that in them shall be light as there always is in nature—more of course in bright pictures, less in low-toned pictures—that therefore the rule of “detail in the shadows” is in a way a good rough-and-ready photographic rule. Yet photographers often stop down their lens and cut off the light, at the same time sharpening the shadows and darkening them, and throwing the picture out of tone. It cannot be too strongly insisted upon that “strength” in a photograph is not to be judged by its so-called “pluck” or “sparkle,” but by its subtlety of tone, its truthful relative values in shadow and middle shadow, and its true textures. Photographers have been advised by mistaken craftsmen to spot out the “dotty high lights” of an ill-chosen or badly-rendered subject to give it “breadth.” Such a proceeding of course only increases the falsity of the picture, for the high lights, as we have shown, are never high enough in any picture, and if a man is so unwise as to take a picture with “spotty lights,” he is only increasing his display of ignorance by lowering the high lights, which are already not high enough. This does not of course apply to the case where a single spot of objectionable white fixes the eye and destroys harmony, but to the general habit of lowering the high lights in a “spotty” photograph. Spotty pictures in art as well as in nature are abominations to a trained eye, and it is for that very reason that such subjects are more common among photographers who are untrained in art matters than in the works of even third-rate painters. The effect of the brightest sunlight in nature, for reasons explained, is to _lessen_ contrast, the effect of a sharply-focussed, stopped-down photograph is to _increase_ contrast in the subject and thus falsify the impression. As the tendency of “atmosphere” is to grey all the colours in nature more or less, and of a mist to render all things grey, it follows that “atmosphere” in all cases helps to give breadth by lessening contrast, as it also helps to determine the distance of objects. As shown in the previous chapter, this aërial “turbidity,” by which is meant atmosphere, takes off from the sharpness of outline and detail of the image, and the farther off the object is, the thicker being the intervening layer of atmosphere, the greater is the turbidity _cæteris paribus_, therefore from this fact alone objects in different planes are not and should not be represented equally sharp and well-defined. This is most important to seize—as the prevalent idea among photographers seems to be that all the objects in all the planes _should be sharp at once_, an idea which no artist could or ever did entertain, and which nature at once proves to be untenable. The atmosphere in the main rules the general appearance of things, for if this turbidity be little, objects look close together, and under certain other conditions are poor in quality.

[Sidenote: Drawing and Lighting.]

In addition to tone and atmosphere, the diminished drawing of objects as they recede from us (mathematical perspective) helps to give an idea of distance, but by choosing a suitable lens, which does our drawing correctly, we need not regard this matter of drawing. A minor aid to rendering depth is the illumination of the object, a lateral illumination giving the greatest idea of relief, but the photographer should be guided by no so-called “schemes of lighting,” because, for more important reasons, it maybe advisable to choose a subject lighted directly by the sun, or silhouetted against the sun. All depends on what is desired to be expressed. For example, an artist may wish to express the sentiment and poetry of a sunset behind a row of trees. Is he to consider the minor matter that there will be little relief, and it is not a good “scheme of lighting”? No, certainly not, otherwise he must forgo the subject. Nature ignores all such laws. The only law is that the lighting must give a relatively true translation of the subject expressed, and that a landscape must not be lighted by two or more suns. In portrait work, even, it must be remembered that the aërial lighting must stand out against the background, for in all rooms there is a certain amount of turbidity between us and distant objects.

[Sidenote: On the Impression.]

The reason we prefer pictures which are not too bright lies in the fact that the eye cannot look long at very bright paintings without tiring. As a physical fact, too, the most delicate modelling and tonality is to be obtained in a medium light. From what has been previously said, it will now be understood that a picture should not be quite sharply focussed in any part, for then it becomes false; it should be made just _as sharp as the eye sees it and no sharper_, for it must be remembered the eye does not see things as sharply as the photographic lens, for the eye has the faults due to dispersion, spherical aberration, astigmatism, aërial turbidity, blind spot, and beyond twenty feet it does not adjust perfectly for the different planes. All these slight imperfections make the eye’s visions more imperfect than that of the optician’s lens, even when objects in one plane only are sharply focussed, therefore, except in very rare cases, which will be touched upon elsewhere, the chief point of interest should be slightly—very slightly—out of focus, while all things, out of the plane of the principal object, it is perfectly obvious, from what has been said, should also be slightly out of focus, not to the extent of producing _destruction of structure_ or fuzziness, but sufficiently to keep them back and in place. For, as we have been told, “to look at anything means to place the eye in such a position that the image of the object falls on the small region of perfectly clear vision, ... and ... whatever we want to see, we look at, and see it accurately; what we do not look at, we do not, as a rule, care for at the moment, and so do not notice how imperfectly we see it.” Such is the case, as has been shown, for when we fix our sight on the principal object or _motif_ of a picture, binocular vision represents clearly by direct vision only the parts of the picture delineated on the points of sight. [Sidenote: Rule for focussing.] The rule in focussing, therefore, should be, focus for the principal object of the picture, but all else must not be sharp; and even that principal object must not be as perfectly sharp as the optical lens will make it. It will be said, but in nature the eye wanders up and down the landscape, and so gathers up the impressions, and all the landscape in turn appears sharp. But a picture is not “all the landscape,” it should be seen at a certain distance—the focal length of the lens used, as a rule, and the observer, to look at it thoughtfully, _if it be a picture_, will settle on a principal object, and dwell upon it, and when he tires of this, he will want to gather up _suggestions_ of the rest of the picture. If it be a commonplace photograph taken with a wide-angle lens, say, of a stretch of scenery of equal value, as are most photographic landscapes, of course the eye will have nothing to settle thoughtfully upon, and will wander about, and finally go away dissatisfied. But such a photograph is no work of art, and not worthy of discussion here. Hence it is obvious that panoramic effects are not suitable for art, and the angle of view included in a picture should never be large. [Sidenote: The Pseudo-Impressionists.] It might be argued from this, that Pseudo-Impressionists who paint the horse’s head and top of a hansom cab are correct, since the eye can only see clearly a very small portion of the field of view at once. We assert, no, for if we look in a casual way at a hansom cab in the streets, we only see _directly_ the head of the horse and the top of the cab, yet, indirectly, that is, in the retinal circle around the _fovea centralis_ we have far more suggestion and feeling of horse’s legs than the eccentricities of the Pseudo-Impressionist school give us, for in that part of the retinal field indirect vision aids us. The field of indirect vision must be _suggested_ in a picture, but subordinated. But we shall go into this matter later on, here we only wish to establish our principles on a scientific basis. Afterwards, in treating of art questions, we shall simply give our advice, presuming the student has already studied the scientific data on which that advice is based. All good art has its scientific basis. [Sidenote: Sir T. Lawrence.] Sir Thomas Lawrence said, “Painting is a science, and should be pursued as an inquiry into the laws of nature. Why, then, may not landscape painting be considered as a branch of natural philosophy, of which pictures are but experiments?”

[Sidenote: Fuzziness.]

Some writers who have never taken the trouble to understand even these points, have held that we admitted fuzziness in photography. Such persons are labouring under a great misconception; we have nothing whatever to do with any “fuzzy school.” Fuzziness, to us, means _destruction of structure_. We do advocate broad suggestions of organic structure, which is a very different thing from destruction, although, there may at times be occasions in which patches of “fuzziness” will help the picture, yet these are rare indeed, and it would be very difficult for any one to show us many such patches in our published plates. We have, then nothing to do with “fuzziness,” unless by the term is meant that broad and ample generalization of detail, so necessary to artistic work. We would remind these writers that it is always fairer to read an author’s writings than to read the stupid constructions put upon them by untrained persons.