Part 74

These are only a few of the instances in which actions of this kind have been observed. It is remarkable that the order of the first two columns in this table may be inverted without changing the result. Thus, instead of exposing iodide of silver to the light and developing the image with gallic acid, one may expose a paper saturated with gallic acid solution, and develop with iodide of potassium and nitrate of silver. The first reaction noted in the table deserves some remark: it is not peculiar to paper, but is common to most organic materials, such as albumen, collodion starch, fabrics, and indeed to organic matters in general, provided they are not of a black colour. Tartaric acid, sulphate of quinine, and nitrate of uranium increase this sensibility. The paper which has been impressed preserves its undeveloped image for a prolonged period if kept in darkness; and it has been found that one piece of paper can impart the image to another by simple contact in the dark. What is still more remarkable, the invisible impressions on a piece of paper may be transferred to another not in contact by merely placing it opposite the first, and separated by an interval of a quarter of an inch. No satisfactory explanation of these phenomena has been advanced, but many conjectures have been made. One of these supposes that some unknown intermediate products are formed, which are, in the case of the latent image on paper, very oxidizable; but in the case of silver salts, &c., very reducible, so that the addition of a silver salt in the first case, and of organic matter in the second, only completes the phenomena by ordinary chemical action. Niepce de Saint-Victor, however, found that a surface of freshly broken porcelain alone will receive a latent impression from light, and will reduce in those places sensitive salts of silver. He believes that the light in these latent images is simply stored up, and that its energy remains fixed to the surfaces until the occasion of its producing a chemical action.

When a pure solar spectrum is made to fall upon paper rendered sensitive by silver salts, the effect is observed to be greatest near the Fraunhofer line H (No. 1, Plate XVII.), and it is prolonged with decreasing intensity beyond the violet end of the spectrum, while towards the other end it terminates about the line F. When other sensitive substances are used, the range of photographic power in the spectrum is modified. It has been found that when a daguerrotype plate which has been impressed by the light in the camera is afterwards exposed to the red or yellow rays of the spectrum, it loses its property of condensing the mercurial vapours. This destruction of photographic impression by red or yellow light has a practical application of great importance, for it permits the processes of preparing paper and plates to be carried on in a laboratory lighted by windows having yellow or red, instead of the ordinary colourless, glass. Thus we see that it is by no means the whole of the solar rays which are concerned in producing photographic images; nay, there are some which even tend to destroy the impressions produced by others. The fact that it is not the light, but only certain rays in the sunbeam, may be proved very conclusively by an experiment with a glass bulb filled with a mixture of equal volumes of hydrogen and chlorine gases. When such a bulb is exposed to the light of the sun or of burning magnesium, which is made to reach it by passing through a piece of _red_ glass, no explosion takes place; but if the bulb be covered only with a piece of _blue_ or _violet_ glass, the explosion is produced just as quickly as if it were exposed to the unaltered rays.

The visible spectrum obtained in the experiment described on page 318 is far from constituting the only radiations which reach us from the sun. For invisible beams of heat, less refrangible than the red rays, are found beyond the red end of the spectrum; and another invisible spectrum stretches far beyond the violet end, formed of rays recognized only by their chemical activity. It is these which effect photographic actions, and though they are in part more highly refrangible than any of the rays producing the visible spectrum, a large portion are refracted within its limits, so that the maximum of photographic action in a spectrum is usually near the violet end. When we wish to examine the spectrum of the heat rays, it is necessary to replace the glass prism by one made of rock salt, for glass absorbs these heat rays. It also intercepts a great part of the most refrangible rays; for when a prism of _quartz_ is substituted for the glass one, the spectrum becomes greatly extended at the violet end. The dark Fraunhofer lines which cross the visible spectrum are represented also in great numbers in the invisible spectrum: in photographs of the _ultra-violet_ rays more than 700 dark lines have been counted. It has been proposed to employ quartz lenses in the photographic camera; but there is reason to believe that the increased transparency of such lenses for the chemical rays would be counterbalanced by certain disadvantages attending the use of quartz.

The beauty of the images which are formed in the camera obscura long ago gave rise to the desire of fixing them permanently. We know how perfectly photography has already satisfied that desire, so far as the _forms_ are concerned. The very perfection of the results obtained in this direction increases our regret at our inability to fix also the colours, and secure the picture, not in grey or brown tones of reduced silver, but with all the glowing hues of nature. An observation made by Herschel, Davy, and others, seemed at one time to hold out hopes of a possible realization of chromatic photographs. It was noticed that the images developed upon chloride of silver, of the different parts of the solar spectrum, partook somewhat of the colours of the rays which produced them. Edmond Becquerel made a plate of polished silver, placed in dilute hydrochloric acid, form the positive pole of a battery. The plate thus became coated with an extremely thin layer of chloride of silver, which, as its thickness augmented, exhibited the series of colours due to the action of light on thin films. The operation was stopped when the plate had become of a violet colour for the second time; it was then washed, dried, polished with the finest tripoli, and heated to 212° F., the whole of these operations having been carried on in the dark. When this plate was exposed for about two hours to the solar spectrum, fixed by proper appliances which counteracted the apparent motion of the sun, the luminous rays were found to have impressed the plate with their respective colours. The yellow was somewhat pale, but the red, green, and violet were exhibited in their true tints. A theoretical explanation has been advanced, which supposes that yellow light, for example, renders the surface of the plate on which it falls peculiarly capable of receiving and transmitting vibrations corresponding to those of yellow light. Just as a stretched cord responds to its own musical note, the modified plate gives back, out of all the vibrations which fall upon it in ordinary light, only those of which it has itself acquired the periodicity. But since the plate has not lost its sensitiveness to take on other rates of vibrations, it receives other impressions, which first weaken and then overcome the former, and, therefore, the colour necessarily vanishes. This kind of difficulty seems to be a necessary concomitant of every attempt in this direction; and all the hopes founded on results yet obtained have been disappointed by the rapid fading of the images.

The comparative cheapness and convenience of Talbot’s process, and especially the facilities which it afforded for the multiplication of proofs, gave an immense impulse to photographic art. But the irregular and fibrous structure of paper prevented the attainment of the beautiful sharpness of outline and clear definition of detail which the plates of Daguerre presented. Sir John Herschel suggested the use of glass plates coated with sensitive photographic films, and Niepce de Saint-Victor succeeded in fixing upon glass layers of albumen (white of egg) containing the silver salts, a method which is still used to some extent. The art received, however, its greatest stimulus from the improvements which ensued on the application of _collodion_ to this purpose. Collodion (κολλα, glue; in allusion to its adhesiveness) is the name which has been given to a solution in ether of gun-cotton, or of a substance nearly allied to it. Its employment was suggested by Le Grey of Paris, but the late Mr. Archer was the first to carry the idea into practice, and the process which he described in “The Chemist,” in 1851, is virtually that which is now almost universally adopted. This process has now been tested, for nearly a quarter of a century, by the united experience of photographers all over the world, and it is agreed that it is surpassed by no other, for it secures every quality which a photograph can possess.[13] The minor details of the method can be, and are, infinitely varied; scarcely two experienced photographers will be found working the process in identically the same manner throughout. Before giving an outline of the collodion process, it may be well to say something respecting the chief instrument of photography—the camera.

Footnote 13:

(1875) But see below, page 541.

The ordinary photographic camera is almost too well known to require description. In its simplest form, Fig. 308, it is merely a rectangular box, in front of which is placed the lens, which slides in a tube, that its position may be adjusted so as to bring the rays to a focus on the surface of a piece of ground glass at the opposite end. This glass is fitted into a light frame, which slides in grooves, so that it can be raised vertically out of its position, and replaced by another frame, B, which contains a recess for the reception for the sensitive plate, and a sliding screen which protects it from light until the right moment. When this frame is placed in the camera, the sensitive surface occupies the same position as that of the ground glass, and the sliding screen is drawn up the moment before the operator removes from the front of the lens a cap which he places there after adjusting the focus. The sliding screen is usually made with a narrow strip at the lower part, joined to the rest by a hinge, so that when it has been drawn up it may be retained in its position, and placed out of the way, by being folded down horizontally. There is commonly provision for two plates in one frame, the slides, &c., being doubled, and the plates placed back to back, as shown at B, Fig. 308. The camera is usually made in two parts, as shown in the figure, that at the back sliding within the other, so that a wider range for adjustment is obtained, and the same camera may even be used with lenses of different focal lengths. Many improvements have been made in the camera, by which it has been rendered more portable, and capable of more adjustments to suit varying circumstances. Fig. 309 represents a “bellows” or folding camera, which appears to supply every requirement for the studio. It is copied from Messrs. Negretti and Zambra’s catalogue, as are also the other figures of photographic apparatus here given. Fig. 307 represents a camera for taking stereoscopic views, fitted with two lenses, so that the two views are taken simultaneously on one plate.

No piece of apparatus used by the photographer is of so much importance as the lens; for good pictures cannot be obtained without well-defined, sharp images on the sensitive plate, and these images must have sufficient intensity to produce the required amount of chemical action in a short space of time. The formation of an image by means of a lens which is thickest at the centre is tolerably familiar to everybody; for most persons must have noticed that the lens of a pair of spectacles, or of an eye-glass, will produce an inverted image of the window-frame on a sheet of white paper, held a certain distance behind the lens. But the diagrams by which the paths of the rays are usually represented seem to convey a false impression to an ordinary reader, who usually goes away with the idea that somehow three rays are sent off by the object, and that one goes through the middle of the lens, and the other two meet it and produce an image. Let us suppose that, by means of a circular eye-glass, the image of a window is projected on a piece of white paper: a straight line passing through the centre of the glass perpendicular to its plane will meet the window and image each at a certain point. The point in which it meets the image is the _focus_ of _innumerable_ rays, which issue from the point in the window; that is, of the whole light sent out in every direction by the point a certain portion falls upon the lens, and by the refraction it undergoes in passing through it, the rays are again brought together at the point in the image. Thus the original point in the object is the apex of a solid cone of rays (if we may say so), of which the lens is the base, and the point in the image is the apex of another cone, having also the lens as its base. These cones would be termed _right_ cones, because their bases are perpendicular to their _axes_, or central lines. But they represent the rays from only _one point_ of the object. Let us now consider how the image of another point is formed, say one in the highest part of the object which forms an image on the screen. Those rays which are sent out by this point, and fall upon the lens, form now an _oblique_ cone, of which the lens is the base, and the central ray will pass through the middle of the lens and continue its journey on the other side with little or no change of direction, forming also the axis of another oblique cone, constituted of the refracted rays, all of which will meet together at the lowest part of the image. Similar cones of incident and refracted rays, all having the lens as base, and all of them cones more or less oblique, will be formed by the light from each point of the object. Thus, the rays which issue from each point are brought together again in a series of points which have the same position with regard to each other, and collectively form an inverted image.

On carefully looking at the image, say of a window-frame, formed by a simple lens, the reader will observe two defects. The first is that the image cannot be made equally clear and well defined at the centre and at the edges: the adjustment which gives clear definition of one part leaves the other with blurred outlines. The second defect, which is best seen with large lenses, consists in coloured fringes surrounding the outlines of the objects. This depends upon the unequal refrangibility of the various rays, but it is obviated in _achromatic_ lenses, which are formed of two or more different kinds of glass, so adapted that the refracting power of the compound lens is retained, and the most powerful rays of the spectrum are brought to a common focus. Such are the lenses always used in the photographic camera, and the skill of the optician is taxed to so combine them as to obtain, not only the union of the principal rays in one focus, but the greatest possible flatness of field in the image, the largest amount of light, the widest angle without distortion of the picture, and other qualities.

Photographers have even been so fastidious in the matter of lenses as to require all the perfection of finish which is given to the object-glasses of astronomical telescopes. Mr. Dallmeyer has made photographic lenses which cost upwards of £250; but it is doubtful whether the pictures formed by these would show any marked superiority over those produced by lenses costing only one-fifth of that amount. Fig. 310 shows the construction of the combination usually employed for taking photographic portraits. A is a section showing the forms and positions of the different lenses; B is an external view of the brass mounting of the lens. It is provided with a flange, C, which is attached by screws to the woodwork of the camera; and within the short tube, of which this is a part, slides the tube carrying the lenses, being furnished with a rack and pinion moved by the milled head, E. D is a cap for covering up the front of the sliding tube. A slit in the tube admits of plates of metal, perforated with circular openings, being inserted. The openings are of various sizes; and these “stops” or diaphragms enable the operator to regulate the amount of light; and to cut off when required the rays passing through the marginal parts of the lens.

It now remains to describe in a few words a method of photography which was, and still is, much practised, namely, the _collodion process_. The collodion solution is prepared by dissolving one part of pyroxylin (gun-cotton) in ninety parts of ether and sixty of alcohol. The pyroxylin for this purpose may be obtained by steeping cotton-wool for a few minutes in a mixture of nitre and sulphuric acid, with certain precautions which need not here be mentioned. To the solution of collodion is added a certain quantity of iodide of potassium, or of iodide of ammonium; and sometimes other substances also are mixed with the solution with a view of increasing the sensitiveness of the plate when ready for exposure. Some of the collodion solution is poured on a well-cleaned plate of glass, which is placed horizontally; it spreads over the plate, and the excess having been poured back into the bottle, the evaporation of the liquids leaves the glass covered with a thin uniform transparent film, which firmly adheres. The next operation is to render the plate sensitive by means of the “silver bath.” This is a neutral solution of nitrate of silver, one part to fifteen of pure water, which is placed in a trough of glass or porcelain, Fig. 311. By the aid of a proper support the plate is introduced quickly and steadily into the solution, immediately after the collodion film has been formed on its surface. In two or three minutes the layer of collodion becomes impregnated with iodide of silver, and when taken out of the bath, the plate exhibits a creamy-looking surface. The operation of sensitizing the plate by the silver bath must be performed in a room to which no light has access, except that which has passed through _red_ or _yellow glass_, or a semi-transparent yellow screen.

The plate is now ready for immediate exposure in the camera. It is placed in the dark slide, in which it is conveyed to the camera; and there the image of the object is allowed to fall upon it for a time, which varies, according to the intensity of the light and the nature of the object, from 3 seconds to 45 seconds. The slide is withdrawn from the camera, and taken again to the “dark” room, _i.e._, where only _yellow_ or _red_ light can reach it. If the plate be now examined, it will be found to present no trace of an image. A latent one, however, exists; and it is developed by pouring over the plate a solution of pyrogallic acid—one part to 480 of water, with commonly a little alcohol and acetic acid added. When it is desired to intensify the image still more, a few drops of the nitrate of silver solution is added to the _developing solution_ immediately before pouring it on the plate. When the picture has become sufficiently distinct, it is washed with pure water, and then immersed in a strong solution of hyposulphite of soda. The last operation is termed by photographers “fixing” the picture, and the substance employed in it is invaluable to the art. It acts as a ready solvent of all the salts of silver which remain on the plate; and the discovery of this property of the hyposulphites by Sir J. Herschel, in 1839, marked an era in photography. The picture is then thoroughly washed in cold water, in order that the hyposulphite of soda may be entirely dissolved out. It is then dried, warmed before a fire, and finally the film is covered with a coat of transparent varnish, by which it is protected from mechanical injury. The image here is _negative_—that is, the strongest lights of the object appear as the darkest tints in the picture, and _vice versâ_. From it any number of _positive_ pictures may be obtained by means of the sensitive paper prepared with chloride of silver as in Fox Talbot’s plan.

As it is a tedious, and perhaps, in some cases, an impossible operation to completely remove all traces of silver salts and hyposulphites from photographs, they have frequently been found to fade; but this is rarely the case with well-prepared specimens. Processes have, however, been devised by which absolute permanence is secured for the photograph. One of the best of these is known as the Carbon Printing Process, and, as improved by Mr. Swan, it is thus practised:

A solution of gelatine is coloured by the addition of Indian ink, or any other pigment which will give the desired tone. This solution is spread over sheets of paper which are then dried. In this condition the paper may be preserved for any length of time without any special precautions. When it is required for use, it is floated, with the gelatine-covered side downwards, in a solution of bichromate of potash, and then dried; but these operations must be carried on in the dark. The paper is exposed under a negative photograph, with which its prepared side is in contact. The effect of the light is to render insoluble the gelatine on all those parts on which it has fallen, and this action extends to a depth in the layer proportionate to the intensity of the illumination. The object is, therefore, to wash away all the _soluble_ gelatine and the colour with which it is mixed; but this soluble gelatine is mainly on the side of the film which is in contact with the paper. The gelatine surface is therefore made to adhere to another piece of paper by means of some substance insoluble in water; and when this has been done, the whole is immersed in warm water. Then the soluble gelatine is soon dissolved; the first paper floats off, and the insoluble gelatine, holding the Indian ink or other colouring matter in its substance, remains attached by the cement. As the thickness of the layer rendered insoluble is in proportion to the intensity of the light passing through each part of the negative, the picture will be presented in all the proper gradations of light and shade.