Read the book: «Nature's Teachings», page 20

The principle of the Stereoscope is now applied to the best microscopes, and its value is incalculable, especially when low powers are used, i.e. those of not less than half an inch focus. The real beauty of many objects could never have been appreciated but for this discovery, nor their true form defined.

On the left hand of the illustration is shown the combining power of the eyes. Supposing the right eye only to be brought to bear upon the little cylinder, only one side of it will be seen, and it looks nearly flat. The same is the case with the left eye. But, when both eyes are used together, both sides of the cylinder are presented to the mind, and thus we get the effect of solidity.

The Stereoscope is so formed, by means of lenses, that the two figures become combined into one, the rays of light being turned out of their course by the arrangement of the glasses.

The Stereoscope, however, although a useful assistant to the vision, is not necessary. It is perfectly possible to combine the two figures without any stereoscope, and to do so merely by squinting, if we may so call it, at the figures. The power of combination is gained with a very little practice, and in a short time the observer will be capable of producing stereoscopic effects without needing a Stereoscope. This ability is very useful when inspecting photographs in a shop-window. Of course the figures are not so much enlarged as they are with the stereoscope, but they are nevertheless quite as clear and well defined.

There is an instrument called the Pseudoscope, which, as its name imports, gives a false idea as to the nature of the object which is viewed through it, converting hollow objects into solid, and vice versâ. The following description of its effect is given by Wheatstone:—

“When an observer looks with the pseudoscope at the interior of a cup or basin, he not unfrequently sees it at first in its real form; but by prolonging his gaze he will perceive the conversion within a few minutes; and it is curious that, while this seems to take place quite suddenly with some individuals, as if the basin were flexible, and were suddenly turned inside out, it occurs more gradually with others, the concavity slowly giving way to flatness, and the flatness progressively rising into convexity.

“Not unfrequently, after the conversion has taken place, the natural aspect of the object continues to intrude itself, sometimes suddenly, sometimes gradually, and for a longer or shorter interval, when the converse will again succeed it—as if the new visual impression could not at once counteract the previous results of recent experience. At last, however, the mind seems to accept the conversion without further hesitation; and after this process has once been completely gone through, the observer, on recurring to the same object, will not find it possible to see it in any other than its converted form, unless the interval should be long enough to have allowed him to forget its aspect.

“Vagaries, however, sometimes occur in these experiments of which it is difficult to give any certain explanation, but which would be probably found referable to the same general principle, if we were acquainted with all the conditions of its operation.”

The Multiplying-glass

Still more extraordinary examples of the combining power of vision are to be found in the eyes of spiders and insects, more especially when we compare them with the work of man. If we take a common Multiplying-glass, such as is shown in the figure, and look at a flower or other object through it, we see the object repeated as many times as there are different foci of vision in the instrument.

Now, taking for example the eyes of a Spider, it would be natural to suppose that the same result would occur, especially as the foci of the eyes point in different directions. The left-hand figure in the illustration represents the eight eyes of one of our common Spiders, belonging to the genus Clubiona, which may be found in almost any outhouse, sitting in its curious web, and ready in a moment to run for safety into its silken tunnel.

It will be seen that the foci of all the eyes are in different directions, and so placed as to command a large radius. Observers have remarked that the eyes are placed in Spiders so as to suit their habits. “Those spiders,” writes Professor Owen, in his “Comparative Anatomy,” “which hide in tubes, or lurk in obscure retreats, either underground or in the holes or fissures of walls or rocks, from which they emerge only to seize a passing prey, have their eyes aggregated in a close group in the middle of the forehead, as in the Bird-spider, the Clotho, &c.

“The spiders which inhabit short tubes, terminated by a large web, exposed to the open air, have the eyes separated and more spread upon the front of the cephalothorax.

“Those spiders which rest in the centre of a free web, along which they frequently traverse, have the eyes supported on slight prominences, which permit a greater divergence of their axis; this structure is well remarked in the genus Thomisa, the species of which live in ambuscade in flowers.

“Lastly, the spiders called Errantes, or Wanderers, have their eyes still more scattered, the lateral ones being placed at the margin of the cephalothorax.”

Yet, although each eye produces a separate image, it is clear that upon the mind of the Spider only a single idea can be impressed, for that otherwise all would be confusion. There must, therefore, be some mechanism in the structure of the eye, the nature of which we are not as yet able to understand.

A still more remarkable instance of a natural Multiplying-glass may be found in the eyes of many insects.

The form of multiplying-glass shown in the accompanying illustration is probably familiar to most of my readers. It consists of a convex piece of glass, cut into a number of facets, and showing in each facet a distinct and separate image of the object to which it is directed. Now, the compound eyes of insects are constructed on much the same principle, except that the number of facets is infinitely more. Taking, for example, the eyes of the Tortoise-shell Butterfly, we find that there are about seventy thousand lenses or facets. Now, it is possible, with care, to remove the eye from the insect, cleanse it, and arrange it in ä microscope in such a way that objects can be seen through it. When this is done, a separate image is seen in each facet, just as is the case with the Multiplying-glass, only, as the facets are very much more numerous, the effect is proportionately more striking.

The reader may notice that the facets of the insect eye appear to be hexagons as perfect as those of the honey-comb. This appearance is probably due to the fact that each eye is covered with a convex plate of glassy brightness and transparency, and that, when such objects are viewed from the front, they appear to have hexagonal instead of rounded outlines. A familiar example of this fact may be found in the glass tumblers which are ornamented with rounded projections on their surface. If a photograph of one of these tumblers be taken, the resemblance to the hexagonal markings of the insect eye is so close that the tumbler might easily be taken for the eye.

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.

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.

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.

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.

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.

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.