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Defective Vision-Remedies.

distinctly apart The parties had not been clearly aware of the fact, but it was easily proved thus :-Two lines of equal strength or breadth are drawn on paper directly crossing each other. If such cross be then held before the eyes, one line being vertical and the other horizontal, the horizontal line appears thicker and darker than the other. The explanation is, that the displacement to one side (generally the right side) of the faint image of the cross leaves the horizontal lines of both still coinciding, and therefore dark, while the vertical lines are separated, and therefore appear broader and less dark. To a person having this defect, a printed page seems to have double letters, and if the lateral displacement amounts to only half the breadth of a common letter, the faint downward lines of the one set appear between the stronger lines of the other, and darken all; but if the displacement be greater, the shadowy lines may coincide with the stronger, and so are, in great part, concealed, except at the very end of the lines. The same clearing effect may be produced by holding the page farther from the eyes. Happily the common spectacle lenses remedy, to a considerable extent, this defect, as well as the feeble convergence.*

An eye much accustomed to examine near and minute objects may lose something of its pliancy, and become defective when tried at distant things, as that of the miniature painter, the engraver, &c. On the other hand, the old seaman's eye, which has so often and uninterruptedly been directed to the distant horizon, straining to catch the view of an expected sail, or of land, has a power of judging of distant things which surprises, while in regard to small, near things, it experiences deficiency.

830. A man who tries to sce with the eyes under water has very indistinct vision, because the difference of density between water and the eye not being so great as between air and the eye, the bending or refraction of light entering from the water is not so great as to produce perfect images on the retina. Aid would be given in such a case by using very convex spectacles. It is to meet the necessity of the case that the lens of a fish's eye is extremely convex, or almost spherical. The white round ball found in the eye-socket of a boiled fish is the crystalline lens of the fish coagulated or hardened, as white of egg is hardened, during the boiling.

*The adjustment of the lens of the eye differs for horizontal and for vertical lines; the distance of distinct vision being greater for the former than for the latter.

Persistence of Impressions.

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Pouillet, in speaking of the eye, describes it as a perfectly achromatic instrument. This has been denied by some eminent physicists, but it is quite certain that in a healthy state, no object appears to be surrounded with any halo of colour, and, practically speaking, the eye is perfectly achromatic for all the purposes of correct vision.

Any impression of light made upon the retina lasts for about the sixth part of a second. According to some, it is only the eighth or eleventh part of a second. Hence, when the burning end of a stick is made to sweep rapidly across the view, its path appears to the eye to be a long continuous line of light; and if it be made to revolve in a circle six times in a second, as when moved by the hand, or fixed to a turning wheel, that circle will appear to the eye to be a complete ring of fire. A small polished ball of steel on the end of an elastic wire, of which the other end is fixed in a block of wood, when caused to vibrate, similarly forms a line or a curve of light. A harp-string while vibrating as it sounds, appears like a flat transparent riband. Lightning or other meteor darting across the sky, although in fact but a single luminous point, is generally thought of as a long line of light: the term forked lightning has reference to this illusion. The same remark applies in a degree to a sky-rocket in its rapid ascent. Two or more colours painted separately on the rim of a wheel which is made to turn rapidly, appear to a spectator to be these colours really mixed :—it has been explained already how patches of all the colours of the rainbow, when mixed in this way on a turning wheel, form white light. If on one side of a card a little bird be painted, and on a corresponding part of the other side a cage be shown, on then making the card turn rapidly by twisting between the fingers and thumbs threads fixed to its opposite edges, the bird and cage will be seen at once, and the bird will appear to be within the cage.

831. A large class of optical toys depend for their explanation on the sensible persistence of impressions on the retina, the general popular expression "an optical illusion," being commonly employed to explain the whole class.

The Zoetrope, or wheel of life (invented in 1860), is one of the most common and interesting of these. It consists of a cardboard cylinder mounted on a vertical axis, so that it can be whirled round rapidly. A number of slits (twelve or more) are cut at equal distances round the cylinder, and at half its depth. On the inside of the cylinder, is placed a strip of paper having the object to be viewed, depicted in as many different attitudes as there are holes in the

disc, the successive attitudes representing those taken up in the performance of any action, such as rising from a chair, making a somersault, or throwing up and catching a ball. If, when the cylinder is rotated, we look through the slits at the figures, the persistence of the impressions on the retina fuses them all into a living motion with surprising resemblance to actuality.

The Anorthoscope is a somewhat similar contrivance. It consists of two discs, one whirling in front of the other and in an opposite direction. In the front one is a series of radial slits, in the back one a set of distorted figures. When they are set in motion, the figures, viewed through the slits, start into regular proportions.

The Phenakistoscope, as it has been called, consists of a disc having a set of figures painted on an inner circle, and a set of radial slits on the outer concentric circle. On whirling, and looking through the slits at the reflection of the figures in a mirror, they instantly appear to be all alive.

832. A certain intensity of light is necessary for distinct vision, but the degree varies much according to the previous state of the organ. A person passing from the bright day into a shaded room, may for a time fancy himself in almost total darkness, but by persons sitting in the room, and become accustomed to the feeble light, every object is clearly seen. The dawn of morning after the darkness of night appears much brighter than an equal degree of light in the evening. When, as the night falls, lamps or candles are first introduced, their moderate glare is often for a time offensive to the eye; and a similar feeling, but still stronger, when in the morning, bed-room window shutters or close-drawn curtains are suddenly opened. After the repose of night, the sensibility of the eye, when first opened, is often such that a window with its frame, or a dressing-table with a glass, or any other object, first seen in a strong light, will so impress the retina that in closing the eyes the images will appear and remain for some time. To a prisoner after long confinement in a dark dungeon, the full light of day is almost insupportable. A dungeon, which to unaccustomed eyes is utterly dark, still to its long-held inmate may seem feebly illumined. The darkness of a total eclipse after bright sunshine, appears deeper than it really is. The long polar night, which lasts for months, ceases to appear very dark to the inhabitants of the country.

If an eye be directed for a time to a black wafer laid on a sheet of white paper, and be then turned to another part of the sheet, a portion of the paper at that other part, of the size of the wafer, will

Accidental Colours.

603

appear brilliantly illuminated; because the ordinary degree of light from it appears intense to the part of the retina lately receiving almost none. An eye directed long and intensely to any minute object—as when a sailor watches a speck seen in the distant horizon, supposed to be a ship, or when a sportsman on a brown heath, keeps his eye fixed on a bird nearly of the colour of the heath, or when an astronomer gazes long at a little star--has the sensibility of its centre at last weakened, and ceases to perceive the object; but if the axis of the eye be then turned a little to one side of the object, so that an image may be formed only near the centre, the object may be again perceived, and the centre, in the meantime enjoying repose, will recover its power.

833. But the most striking fact connected with the sensibility of the retina is, that if part of it be strongly exercised for a time, by looking at some bright-coloured object, on the eye being then turned away or altogether shut, an impression or image will remain of the same form as the object lately contemplated, but of a different colour, deemed the opposite or complementary colour of the other. Thus if an eye be directed for a time to a red wafer laid on white paper, and be then shut or turned to another part of the paper, a beautifullybright green wafer will be seen; and vice versa, a green wafer will produce a red spectral image, violet will produce a greenish yellow, and yellow a violet, and a cluster of wafers will produce a similar cluster of opposite colours. Then if the hand be held over the closed eyelids to prevent almost entirely the access of light to them, the spectral image of a bright object, lately viewed, will appear luminous surrounded by a dark ground, and when the hand is again removed the contrary will be true. Again, if the eye be considerably fatigued by looking at the setting sun, or even at a window with a bright sky beyond it, or at any very bright object, on then shutting it, the lately contemplated forms will be perceived, first of one vivid colour, and then of another, until perhaps all the primary colours have passed in review. These extraordinary facts prove that the sensations of light and colour, although excitable by light, are also producible without it. This truth gave occasion to Darwin's theory, that the sensation of any particular colour, as red, for instance, is dependent upon a certain state of contraction of minute fibres in the retina,—and that the fibres, when fatigued in that condition, seek relief when at liberty, by throwing themselves into an opposite state, --as a man whose back is fatigued by bending forward, relieves

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Complementary Colours.

himself not by merely standing erect, but by bending the spine backwards.

834. Complementary colours.-The term complementary is derived from the Latin compleo, to fill up. If the seven colours of the spectrum are painted in their due proportions on a disc, the complementary colours correspond to directly opposite parts of the circle. They may be thus set down in order,—

Each colour with its opposite, produces by blending, white light, a fact demonstrated by experiments on polarized light (see p. 690), as well as by the rapid revolution of a card-disc painted with the two colours. Helmholtz has proved that yellow and blue arc complementary (Tyndall). This might seem inconsistent with the well-known fact that blue and yellow pigments undoubtedly produce green, but, as Tyndall observes, the mixture of pigments is totally different from the mixture of lights. Certain solids, liquids, and even mixtures of gases, split white light into its complementary colours and are what are called dichroic, i. e., of two colours. The red colouring matter of blood dissolved in an alkali is green by reflected and red by transmitted light. Other red liquids have a similar property. Gold-leaf reflects a deep red colour, but the transmitted light is greenish coloured. The atmosphere in a large mass reflects a splendid blue (sky-blue), but the light which it transmits, as seen in the rising or setting sun, is yellow or orange. In all these cases of dichroism, the light is actually sifted; that which passes through is the balance or complement of the rays which do not penetrate, but undergo reflection. The two sets of rays constitute white light.

A shadow produced by coloured light, is seen with the complementary colour of that which produces it. Thus the orange-coloured light of the rising or setting sun produces on a white ground a beautiful sky-blue shadow. This, no doubt, proceeds from the reflected light of the atmosphere when the transmitted light is cut off. The light of the moon is a pale greenish blue. The shadows which it produces on a white surface, in the presence of artificial light, are red or reddish coloured.