CHAPTER II.

THE WATER TELESCOPE.--IRIS OF THE EYE.--MAGIC LANTERN.--THE SPECTROSCOPE.--THE THAUMATROPE.

Limits to Sight in the Water.--Effect of a Ripple.--The Eyes under Water.--The Water Telescope, its Structure and Mode of Use.--Gyrinus, or Whirlwig-beetle, and its Double Set of Eyes.--The Iris of the Eye, and its Double Set of Contractile Fibres.--Cotterill’s Lock and its Structure.--The Magic Lantern and its Principle.--Chinese Shadows.--Spectre of the Brocken.--An Adventure in Wiltshire.--Effect of the Halo.--The Spectroscope.--Its Structure explained.--A Star on fire.--Motes in the Sunbeams.--Bessemer Steel made by aid of the Spectroscope.--Absorption Bands.--Detection of Blood.--A Man’s Life saved by the Spectroscope.--The Pocket Spectroscope.--The Rainbow, Dewdrop, Soap-bubble, Opal, and Pearl.--The Thaumatrope.--Structure of the Retina.--Complementary Colours.--The Zoetrope and Chromatrope.--Wheel Animalcules and their Structure.--An Optical Delusion.

THE WATER TELESCOPE.

Every one who has watched the movements of the various creatures which live below the surface of the water is aware how entirely dependent he is on the unruffled character of that surface. No matter how clear the water may be, the least ruffling of the surface will effectually shut out all sight:--

“But if a stone the gentle sea divide, Swift rippling circles rush on every side, And glimmering fragments of a broken sun, Banks, trees, and skies in thick disorder run.”

And there is an end of the observations. If, however, the eyes can penetrate below the surface, the ruffling is of little consequence, so long as the water is clear. Consequently, whenever the top of the bank is sufficiently near the water, it is possible to continue the observations by lying down, and immersing the head above the eyes. This plan, however, is not a very comfortable one, although I have often followed it on a windy day when the surface was too ruffled to permit of vision in any other way.

Still, there is an instrument by which it is possible to counteract the ruffle of the surface, and to see objects with tolerable plainness. This is called the Water Telescope, and it is of very simple construction. Like the ordinary telescope, it consists of a tube, but, instead of the convex and concave lenses of that instrument, it has only a single glass at one end, and that glass is perfectly plane.

When used, the eye is applied to the open end, and the glazed end lowered into the water. The sight is then undisturbed by the ripple, and the effect is the same as if the eyes themselves were lowered beneath the surface.

It is much used in looking for shells, sea-urchins, and other creatures which live in the bed of the sea.

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IN the insect world we have an example of a natural Water Telescope. I do not say that the inventor of the Water Telescope took his idea from the insect, but the reader will see that he might very well have done so.

There are sundry little beetles popularly called Whirlwigs or Whirligigs, and scientifically known by the name of _Gyrinus_. All these names allude to the insect’s habit of whirling about on the surface of the water, with a movement which seems ceaseless and untiring. Allusion has already been made to the Whirlwigs on page 22.

Their object in their perpetual waltz is not so much amusement as food, which chiefly consists of the tiny insects which fall into the water. Now, in order to enable it to see both above and below the water, a peculiar structure is required. Generally the insects possess one pair of compound eyes, each group being set on the sides of the head. In the Gyrinus, however, there are two sets of these eyes, one pair being on the upper surface of the head, and the other on the lower surface. Thus, while it can use the upper pair for seeing objects which are out of the water, the lower pair of eyes, which are submerged, act the part of the Water Telescope, and enable it to see objects that are below the surface. Were it not for this precaution, even the ripples which it makes by its own rapid progress would prevent it from seeing.

THE IRIS OF THE EYE.

I have often wondered, when contemplating the astonishing mechanism by which the Iris of the Eye is able to contract or enlarge the pupil according to the amount of light, whether any similar mechanism would be used in Art. As anatomists know, the Iris is composed of two layers. One consists of radiating fibres, which serve to enlarge the pupil, while the other layer surrounds the latter, and by its elasticity serves to contract it. As any one may see by looking in a mirror and shifting the light, the pupil is perpetually changing its diameter, but always retaining its circular shape. A glance at the illustration will show the two layers, and aid the reader in understanding the mode in which they work.

Some years ago, while looking at the account given by Mr. J. Price of a lock invented by Mr. Cotterill, I saw at once that the inventor, whether consciously or not, had followed the mechanism of the eye, as far as metal could be expected to imitate animal fibre.

In the very centre of the lock there is a small circular opening, resembling the pupil of the eye, and serving to admit the key, just as the pupil admits light. Around this pupil, if we may so call it, are ranged some twenty thin steel slides which move in channels, up and down which they slide. Round the circumference of the lock are a corresponding number of spiral springs, each of which presses on the base of a slide, and forces it towards the centre.

The reader will now see that the radiating slides of the lock represent the radiating fibres of the iris, and that the spiral springs represent the circular fibres. Both perform the same office, the steel slides regulating the size of the aperture, and the spiral springs pressing them all towards the centre. The key of the lock answers the same purpose as does light in the eye, which by its mysterious pressure enlarges or contracts the pupil.

This is not the place to describe this very ingenious lock in detail, but I may state that it has never been picked. Even Mr. Hobbs, who tried it for twenty-four hours, gave it up, and, when he saw the interior mechanism, said that if he had tried for a month he should have made no progress. This is an unconscious testimony to the wisdom of following Nature in Art.

THE MAGIC LANTERN.

We are all familiar with the Magic Lantern, whether it may take the form of the mere child’s toy, be developed into Dissolving Views, or throw black shadows on a curtain, in which case it is called by the name of Chinese Shadows. In all these cases the principle is the same. First we have a light behind the object whose reflection is to be seen. Next we have the object itself, and lastly the surface upon which it is reflected. As to the variety of mirrors, lamps, and lenses which are used to produce different effects, we may put them aside as foreign to our present purpose.

Generally the object is reflected upon a white curtain or sheet, but sometimes, when a specially weird-like effect is needed, a cloud of thick smoke takes the place of the sheet, and upon it the reflection is shown, as seen in the accompanying illustration.

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NATURE has her Magic Lanterns as well as Art, and wonderful things they are sometimes, the well-known Brocken Spectre being an excellent example. It is not, however, necessary to visit the Brocken in order to see this apparition, for I have seen it in perfection in England.

Many years ago, when living in Wiltshire, I went before daybreak to the top of a very high conical hill. The morning mist was so thick that I could scarcely see my way up the hill. When I reached the summit, I stood there for some time, trying to see the landscape, but the mist was so thick that I could barely tell the points of the horizon by the brighter look cast by the coming Day in the east.

I was looking westward, when suddenly the sun rose behind me, and I saw the Brocken Spectre as I have sketched it in the accompanying illustration. It was a gigantic shadow of myself, projected on the mist, just as a Magic Lantern projects the image on a sheet or a smoke-cloud. Of course my gestures were repeated, and it really looked almost awful to see this gigantic spectral figure set in the mist.

Perhaps the most extraordinary part of it was the enormous halo of rainbow colours round the head. No matter where I moved, the halo surrounded the head of the image, its colours being comparatively bright near the centre, and becoming gradually paler towards the circumference.

Another point about this natural Magic Lantern ought to be mentioned.

Wishing to show a friend the extraordinary sight of a Brocken Spectre, I took him up the hill on a misty day like that which has been briefly described. According to surmise, two spectres appeared instead of one, but the halo was not doubled as well as the shadow. I could see my friend’s shadow, and he could see mine. But, although the halo was as bright as before, each of us could only see it encircling his own head. We stood as close to each other as we could, we moved apart as far as the nearly conical top of the hill would allow, and in both cases each of us could only see his own halo.

Perhaps the reader may remember the wonderful spectre-scene drawn by Mr. Whymper, and viewed from the Matterhorn just after the accident which had killed several of his companions in the ascent of the hitherto impregnable peak. In the mist there suddenly appeared three vast dark crosses enclosed in an oval. Considering the highly-strung nerves of the survivors, it was no wonder that they were all shaken by such an appearance, and that the guides were for a time too frightened to proceed.

THE SPECTROSCOPE.

Next we come to one of the most astonishing and beautiful optical instruments ever made by the hand of man. It is called the Spectroscope, because it deals with a certain arrangement of rays which is called a “spectrum.” Many years ago Newton discovered the cause of the lovely colours which deck the rainbow, and the fact that, by passing a ray of white light through a prism, it was decomposed into seven colours, which invariably came in the following order--Red, Orange, Yellow, Green, Blue, Indigo, and Violet. He also discovered that, by looking at that coloured band through another prism arranged in a different manner, the decomposed rays were again brought together, and white light was the result.

Newton had thrown the light on the prism through a round hole, but some time afterwards Dr. Wollaston employed a narrow slit for the purpose, and then found that the spectrum was traversed by dark lines which never changed their places. On these lines depend all the discoveries that have been made by the aid of the Spectroscope. The chief of them are designated by the letters of the alphabet. (See page 300.)

It was soon found out that if burning gases were viewed with the Spectroscope, lines were still seen, but they were bright instead of dark, and that they invariably occupied the place of one or more of the dark lines shown by the spectrum of sunlight. Then it was discovered that these burning gases absorbed or stopped out the light in the solar spectrum, and from that moment the science rapidly advanced.

At the present day the Spectroscope not only determines the metals which exist in the sun, but also those of the fixed stars. It even analyzes the constitution of double stars, and shows the reason why one star should be red and the other green.

One of the most astonishing discoveries in astronomy was due to the Spectroscope.

During the month of May, 1866, one of the stars in the Northern Crown (_Corona Borealis_) was seen to undergo a rapid change. It was originally one of the tenth magnitude, but in a short time increased in size and brilliancy until it nearly equalled Sirius, Capella, or Vega. It remained bright for some time, and then rapidly faded until it resumed its former size.

How this change was effected we never should have known but for the Spectroscope. No sooner, however, was this instrument pointed at the star than there appeared in the spectrum the three well-known lines--red, green, and violet--which denote burning hydrogen. There was no doubt on the matter, and the Spectroscope showed us that we were witnessing a conflagration the like of which was never seen or scarcely imagined.

Supposing our sun, which is known to be one of the stars, and about which there are vast volumes of hydrogen gas, were to blaze out in a similar manner, the result would be that the whole of the planets would be consumed in a few seconds, and converted into gases. In an instant every living thing would be swept off the surface of the earth by this fearful heat, and, as Mr. Roscoe says, “our solid globe would be dissipated in vapour almost as soon as drops of water in a furnace.” So, as Mr. Huggins observes, the old nursery rhyme,--

“Twinkle, twinkle, little star, How I wonder what you are,”--

is no longer tenable, for we really do know the composition of the stars.

The Spectroscope not only tells us the substance of which the sun and the most distant stars are made, but gives us the same information about the “gay motes that people the sunbeam.” It tells us that they are common salt in very minute particles. They have been dashed into the air by the winds as spray, and then dispersed over the whole globe. This is one reason why we have so much salt in our bodies, and why the blood and the tears are so salt.

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IT is also applied to the arts. The well-known Bessemer process consists in pouring melted iron into a peculiarly shaped vessel called a “converter,” and blowing air through it for the purpose of burning out the carbon. From the mouth of the converter issues a volume of magnificent flames, and at a certain moment the skilled workman who directs the process inverts the vessel and pours out the steel. A very few seconds too soon or too late would spoil the whole of the metal, in the former case it being simply brittle cast-iron; and, in the second, becoming so thick that it could not be poured out.

Only a few workmen could judge rightly the exact point at which to shut off the air-blast. They watched the flame, and by some change in it, too slight to be noticed by any except experienced eyes, knew the moment when the iron was converted into steel.

Such men could, of course, demand any wages they liked, and, by striking, stop the whole works. The Spectroscope, however, performed this delicate discrimination far better than the best workman. When directed to the flame, the bright lines indicating carbon are seen in the spectrum. When the blast has continued for some twenty minutes, the carbon lines suddenly disappear, showing that the carbon has been burned out, and giving to the workman the signal to shut off the air-blast.

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ANOTHER discovery was, that liquids gave dark lines, technically termed absorption bands, of different widths and in different parts of the spectrum. Even liquids which had no perceptible colour threw bands as bold as those which were coloured, while coloured liquids threw totally different bands, irrespectively of their own colour.

For example, the green colouring matter of leaves, called chlorophyll, throws a single broad band on the extreme left--_i.e._ across the red part of the spectrum--so far back, indeed, that it is not easily seen at first.

Then, suppose that we make some pale solutions of red substances, such as carmine, magenta dye, port wine, logwood, permanganate of potash, and blood, it is possible to have them so exactly resembling each other that not even the microscope can discriminate between them; yet the Spectroscope instantly detects the colouring matter of each solution.

The instrument is, therefore, invaluable in detecting adulterations of wine. For example, supposing that red wine is suspected of owing its redness to logwood, and not to the genuine grape, a drop is mixed with water and viewed through the Spectroscope, which instantly tells whether the colouring matter is grape or logwood. And as, by photography, the spectrum can be exactly copied, an indelible record is procured of the true nature of the object.

So marvellously delicate is the instrument with regard to blood, that it detects the thousandth part of a grain of colouring matter in a blood-stain.

If upon the spectrum were printed the word BLOOD in the largest and blackest of capitals, it could not be more legible to an ordinary reader than are the two blood-bands to the eye of a spectroscopist. There is nothing like them in nature, and whether it be by association of ideas, or by absolute fact, these two bars have a strangely menacing look about them. Not only that, but if the blood should be that of a person suffocated with carbonic acid gas, the Spectroscope will say so.

Some years ago a man owed his life to the Spectroscope. A mysterious murder had been committed, and the police had arrested a man who was found near the spot. He could give no intelligible account of himself, and the sleeves of his coat and a part of his waistcoat were deeply stained with a red substance just like clotted blood. A piece of each garment was cut off and given to a well-known spectroscopist, who tried the red matter in the instrument, and at once declared it not to be blood. What it was he had not time to ascertain, so he sent it to a brother in science, who, after examination, pronounced it to be red gum.

By degrees, the man, who had been intoxicated when arrested, stated that he had been to see a friend who was a journeyman hatter. It was then found that he had been leaning on the workman’s board, and so had carried off some of the gummastic with which hats are stiffened. Had it not been for the infallible Spectroscope, the man might have lost his life.

Thus we see that the Spectroscope is the elephant’s trunk of optics, equally fitted for the greatest and smallest, the farthest and nearest, of objects. It is equally at home in earth and sky. When attached to the telescope, it reveals the constituents of the stars, and, when affixed to the microscope, it shows us the colouring matter of a green leaf. It produces the best steel, and detects adulteration in wine. And, lastly, as we have seen, it turns lawyer, and settles the evidence by which the life of a man is lost or saved. It can determine the purity of the smallest coinage, and tell us why a star changes in magnitude.

Yet all these wondrous revelations are made by a few prisms and a magnifying-glass. I possess a Spectroscope, made and presented to me by Mr. J. Browning, the celebrated optician. This astonishing instrument is only three inches long, and half an inch in diameter, so that it can be carried in the waistcoat pocket. I always keep mine in a finger of a white kid glove, which is amply sufficient for it. Yet it gives the spectrum of the sun with its principal lines, will detect the fraudulent wine merchant, and could have decided whether the accused man should be acquitted or hanged.

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MARVELLOUS and mighty as is this engine, it lay concealed in Nature ever since the sun’s rays shone upon earth and a drop of water existed. The Rainbow is nothing but a vast spectrum, a transverse slice of which would be a good representation of the coloured band which is shown in the instrument. It is prefigured in the ever-shifting rainbows of the water-fall and fountain, which latter may even be seen in the fountains of Trafalgar Square, while at the Crystal Palace their beauty has long been noticed.

There is not a dewdrop which is not a miniature Spectroscope, as it glitters with its wondrous iridescence in the rays of the rising sun; there is not an opal with its shifting hues, nor the splendour of the soap-bubble, nor the nacre of the common river mussel or the ormer shell, which does not owe its beauty to the same principles which govern the Spectroscope. Every green leaf, and blue or pink or yellow petal, every varying tint of the mackerel sky, every blaze of sunset and bluegrey of sunrise, owes its beauty to those wondrous laws of light which had been hidden for so many centuries, until they were unveiled by the simple prism of the Spectroscope. As in so many instances, the revelation lay concealed until the coming of the revealer, whose inspired hand raised the dark veil of centuries.

THE THAUMATROPE.

Middle-aged persons will recollect that since the days of their childhood a great variety of optical apparatus has been invented ending in the word “trope.” This is a Greek word, signifying to turn, and is given to the instruments because they revolve.

All these toys--and they may some day become more than toys--depend on a curious property of the human eye. The reader will remember that in the description of the human eye, as compared with the camera obscura as applied to photography, it was mentioned that the image was thrown from the front to the back, and in the one case was received on a naturally sensitive membrane, and in the other on a film rendered artificially sensitive by chemical means. This membrane is called the “retina,” because it not only receives the impression, but retains it for some little time after the object is removed. It has been calculated that the duration of the image is about the eighth part of a second.

Thus the eyelids are perpetually and unconsciously closing and opening with a rapid movement, popularly called “winking.” This movement is for the purpose of cleansing the eyeball, and, were it not for the image-retaining power of the retina, we should pass a considerable part of our time in absolute darkness. As it is, the impression of external objects on the retina lasts longer than the time occupied in winking, and, in consequence, we are not conscious that any interval of darkness has elapsed.

Again, when we have been looking steadfastly at an object, and then move our eyes, the image of that object is seen in the new focus; and it is worthy of notice that such object is always seen in its “complementary” colour. For example, if we have been looking at a scarlet spot, and suddenly move our eyes, we shall see a spot exactly similar in size and shape, but of green.

I well remember that when I was a boy I was reading with almost feverish anxiety the green handbill of a travelling circus, to which I hoped that I might be allowed to attend. Having finished it, I asked for some note-paper, for the purpose of putting my request in writing, but, to my astonishment, mixed, perhaps, with a little irritation, all the paper supplied to me was of a bright pink. For a time no arguments could convince me that the paper was really white, until by degrees the pink hue became paler and paler, and the paper assumed its normal whiteness.

The fact was, that the eye had become saturated with the green--_i.e._ the blue and yellow rays--and could see nothing but their complementary colour, which was pink.

A good example of this property may be found in a lighted stick, which, if rapidly whirled round, appears to form a continuous circle of fire. The reason of this is, that the impression made on the retina by the fiery point does not cease until the stick has again come round in its course.

Then there are those well-known chromatic tops, in which are inserted pieces of bent wire. When the top is spun these pieces of wire assume exactly the appearance of transparent jugs, vases, glasses, and similar articles. A very pretty illustration of this principle is given by a little machine, which is made to revolve rapidly by means of a multiplying wheel.

Upon its surface are fixed little pins, with polished globular steel heads, and, when the handle is turned, these heads form the most beautiful and intricate figures with exact accuracy.

Another toy, called the Thaumatrope, or Wonder-turner, is equally ingenious and beautiful, and is sufficiently simple to be made by any one with a slight knowledge of drawing. A disc of white cardboard is cut, and upon each side of it is portrayed some object. If the disc be caused to revolve rapidly, these two subjects will be seen at the same time, the image of each being held on the retina long enough to allow the other to take its place.

Some very beautiful combinations may be made by means of this instrument. For example, a horse may be on one side, and a man on the other, and, by spinning the disc, the man will be seen mounted on the horse. Then we may have a boat on one side, and a rower with his oars on the other. Similarly a mouse can be put into a trap, or a bird into a cage.

The reader must remember that these subjects must be drawn as if they were upside down with regard to each other, so that the man who is to ride the horse is drawn as if he were standing on his head, and the mouse which is to enter the trap looks as if it were lying on its back.

The most simple manner of spinning the disc is by means of two threads, each being inserted near the edge of the disc, and exactly opposite each other.

A very ingenious modification of the Thaumatrope is made by inserting at one side of the disc two strings, of which one is elastic. It is evident, then, that by lengthening or shortening the elastic string, the axis can be changed, and the objects on the opposite sides placed in positions relatively different from each other. Thus the jockey may be made to jump on and off his horse, the bird to go in and out of its cage, the mouse to enter the trap, and so on. This simple invention allows of infinite combinations, so that a tree may be made to sprout, a man to move his limbs, and a bird to flap its wings. It was invented, I believe, by Dr. Paris, author of “Philosophy in Sport made Science in Earnest.”

On the right hand of the illustration are seen three figures, each representing a means of obtaining an ocular delusion through the principle of which we are now treating.

The lower figure is called the Zoetrope, or Wheel of Life. As the reader may see, it consists of a hollow cylinder, revolving on a centre, and having within it a series of figures. When the wheel revolves, and the figures are viewed through the slits, each figure seems to be in lifelike motion, whence the name of Zoetrope. In the present case the figures are those of boys jumping over posts.

The mode in which this effect is produced is as follows:--Suppose that a boy were really to jump over a post, he would go through a series of motions, and his body be placed in a certain series of positions, before he cleared the post. Supposing, then, that several points were chosen in his course, and his body drawn as it would appear at these points, and the drawings placed in their proper order in the Zoetrope, it is evident that the figures must appear in movement. Before the retina loses the image of the boy standing in front of the post, it takes in that of the boy stooping, with his hands on the top of the post, and so on until he has reached the ground on the opposite side.

Another mode of producing the same effect, called the Phantasmascope, is seen above the zoetrope. In this case the images are placed on the inside of the disc, which is held opposite a mirror, and the figures viewed through the slits.

The last of these figures is the rather complicated one, like the back of an “engine-turned” watch. This is called the Chromatrope, or Wheel of Colour, and is always a favourite object in a magic lantern. It consists of two circular plates of glass, one upon the other, and painted in variously coloured curved lines, as seen in the illustration. When the image is thrown upon a screen, and the glass plates turned in opposite directions, a most singular and beautiful effect is produced. The lines, unless the eye follows them very closely, disappear, and torrents of coloured spots seem to pour from the centre to the circumference, or _vice versâ_, according to the direction in which the glass wheels are turned. So perfect is the illusion, that it is almost impossible to believe that the movement is only circular, and not spiral.

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NOW we will pass from Art to Nature. The figure on the left hand of the same illustration represents part of one of the Wheel Animalcules, so called because they look exactly as if the fore-part of their bodies were furnished with two delicate wheels, running rapidly round, and evidently moving or stopping at the pleasure of the owner.

Soon after the powers of the microscope became known, these Wheel-bearers were discovered, and for a long time they were thought to have a pair of veritable revolving wheels upon their heads. They were naturally held in high estimation, as, although almost every kind of lever can be found in the animal world, a revolving wheel had never been seen. However, as the defining powers of the microscope improved, the so-called wheels were found not to be wheels at all, but stationary organs, and that their apparent revolution was nothing but an optical delusion.

The wheels are, in fact, two discs, around the edges of which are set certain hair-like appendages, called “cilia,” from a Latin word signifying the eyelashes. Each of the cilia has an independent motion of its own, and, as they bend in rapid and regular succession, they produce an effect on the eye similar to that of a revolving body. As for the animal itself, they produce a double effect, either acting as paddles, and forcing the animal through the water, or, when it is affixed to some object, causing a current which drives into its mouth the minute beings on which it feeds.

The particular species of Wheel-hearer whose mouth is here shown is called scientifically _Limnias ceratophylli_. It derives the latter name from the fact that it is mostly found on the submerged stems and leaves of the Hornwort (_Ceratophyllum_), which is very common in ponds and slow streams. The creature is, however, to be found on the water-growing plants, and Mr. Gosse, in his “Evenings with the Microscope,” gives a very full and graphic account of itself and its habits.

He specially mentions the use of the wheels, and, by dissolving a little carmine in the water, had the pleasure of seeing the coloured granules swept into the mouth by the current caused by the cilia through the jaws, and so into the stomach.

USEFUL ARTS.