Single Vision with Two Eyes. When we look at an object with both eyes, it appears under ordinary circumstances as a single object. Two images of the object are produced, how ever, one on the sensitive coat (retina) of each eye, though we are not conscious of two images. If now a finger be pressed strongly on one eye ball so as slightly to push it to one side, the object will appear double; when the pres sure is removed the object is again single. It seems, therefore, that single vision with two eyes is produced when the image of an object falls on the corresponding part of each retina. If one eye be so displaced that the image falls on a part of its retina that does not correspond to that of the other eye, then the object ap pears doubler-double vision, as it is called, is produced. In looking from one object to another the eyes are moved together in harmony with one another, and single vision is constantly secured. If, however, a person suffers from paralysis of one of the muscles of one eyeball, then it is evident that when the eyeballs are moved in a particular direction the paralysed muscle will be unable to contract, will be un able to pull the eyeball round in that special direction, and thus, while the eyeball, whose muscles are all sound, is properly directed to the object, the other one cannot be sufficiently brought round, the image of the object will not fall on corresponding points of the two eyeballs, and double vision will result. Since only one muscle is affected, the eyeball can be moved quite freely in every direction but one, and thus in all other directions single vision will be produced, because in all directions but one both eyeballs will act together. Persons who squint would also "see double" were it not that they accustom themselves to use only the straight eye, and speedily become altogether unconscious of the image on the squinting eye.
Single vision with two eyes, that is binocular vision, enables us to judge of the solidity of objects looked at. The image that falls on each eye is not absolutely the same, because each eye regards the object from a very slightly different point of view. The two images, dif fering so little from one another, are fused together in our consciousness; but the result of the slight difference is to give us a particular impression which experience has taught us is due to the object being not flat but raised—we have, that is, the impression of a solid body. This may be illustrated by a very simple ex periment. Fig. 184 shows two views of a cube; the view on the right hand presents the ap pearance that would be perceived suppose a cube were looked at by the right eye, while on the left band is the appearance of the cube to the left eye, the position of the person not being changed. Now let one take a card about 10 inches long and hold it between the two views, let the person rest his forehead on the upper end of the card and look on the figure, so that the left eye sees only the left' view and the right eye sees only the right view ; with a slight converging of the eyes only one cube is seen, but it is neither the right-hand nor left hand view, but a view produced by an over lapping of the two, and the cube stands out from the paper as if it were actually a solid body.
This is the principle of the stereoscope, Fig.
185. Tt is n box divided into two sides by thin partition down the centre. In the bottom of the box is placed a card on which are pasted two photographs of the same thing. Each photograph, however, has been taken from a slightly different point of view, so slightly dif ferent that, without careful examination, one would conclude they were both absolutely the same photographs. The views are of such a
size that each one extends across the space in the bottom of its compartment, and the central partition accurately separates the two. At the opposite end of the box are two lenses, so placed that when the box is held up to the eyes each eye looks through one lens. Looking through the glasses each eye sees a photograph, slightly magnified. The lenses are of such a shape that they cause a slight displacement of the pictures, so that the images fall on corres ponding points of the two eyes. The two images are fused together, and one becomes con scions of only one picture, in which the objects stand out in relief, just as they would appear were one looking at the actual objects them selves.
Colour. Ordinary sunlight appears to be compounded of seven different colours: red, orange, yellow, green, blue, indigo, and violet. If a wedge-shaped piece of crystal—a prism— be held up between the sunlight and the eye, these various colours will be seen, because the prism separates the white light into its consti tuent colours. The band of the different colours produced in this way is called a spectrum—the spectrum of sunlight. The rainbow is such a spectrum. The same thing can be shown in another way. If the seven colours be painted on a wheel as indicated on Fig. 186, and in proper proportion, and the wheel be turned on a pivot through its centre with great rapidity, the eye will not perceive any colour at all, but the wheel will appear a dull white. If, how ever, one of the seven colours be omitted, the revolving wheel will be no longer white. The six remaining colours will still be fused together, so as to give the impression of one colour—the result of the union of the six.
Now bodies appear variously coloured because of their behaviour to white light. Some bodies held up between the light and the eye permit all the rays to pass through, and therefore ap pear without colour, other bodies do not permit all the rays to pass, they intercept some. Thus one body keeps back all rays but the red ; they alone pass through, and thus the object appears red. Another body keeps back all the rays but the yellow and some of the red, and it appears to be orange. A third body permits only the violet rays of white light to pass, and it, therefore, appears violet, and so on. Again another class of bodies do not permit rays to pass through them, but they reflect rays. Their apparent colour depends upon whether they reflect white light unchanged, or whether they reflect only some of the seven different rays of white light and retain or absorb others. A body that reflects the white light unchanged appears white, a body that does not reflect it at all, but absorbs it, appears black. One body absorbs all rays of white light except red, these it reflects and it appears red in consequence. If you throw a beam of red light on such a body, say a piece of red ribbon, it reflects the rays and appears bril liantly red. But throw a ray of yellow light on the red ribbon, it does not reflect but absorbs yellow light; in consequence it appears no longer bright coloured, but almost black. An orange ribbon reflects partly red and partly yellow rays of white light, the others it retains, and it therefore appears orange, that is a blend of red and yellow.