lower.

Further than this, so far as we know how the impressions are conveyed from the retina to the brain, there is no more reason to expect the impression of an inverted picture than that of an erect one.

When one looks at an object, the crystalline lenses must be focussed for the distance of the object, and the optic axes of the two eyes must be directed towards it, the degree of convergence of the axes increasing with the nearness of the object. These two circumstances afford the means of forming an estimate of the distance of the object, when the distance is not very great; but, for great distances, both the focussing of the lens and the convergence of the eyes are practically nil, and thus do not assist in forming an estimate of the distance. Of course, it frequently happens that other circumstances help one in judging a distance, such as seeing objects of approximately known dimensions in the field of view.

Here may be mentioned the errors sometimes made with regard to the sizes of object, when circumstances lead to their distances being wrongly estimated. Thus in a clear mountain atmosphere, distant mountains are thought to be nearer than they are, and consequently are judged to be smaller than they are. And objects seen in a fog sometimes appear to be of enormous dimensions, because, being seen indistinctly, they are thought to be further off than they are.

Binocular Vision. If we look at a group of objects with one eye, the appearance perceived is very much like that of a flat picture of the object, only that the eye can form some estimate of the distances by means of the focussing of the crystalline lens. But if both eyes are used, the case is quite different. We now appreciate much more clearly the solidity or depth of the field; and this is the great advantage of vision with two eyes over vision with one. It must be noticed, in the first place, that, though a picture is formed on each retina, yet only one impression is conveyed to the brain. To each point in one retina there is a corresponding point in the other; and when the two images of an object are formed at corresponding points, as is in general the case, the pictures produced overlap, and only one is perceived. Now, in looking at a solid object with both eyes-for instance, the hand held edgeways in front of the face-it is not quite the same picture that is perceived by the two, because they see it from different points of view. But if it were a flat picture of the object that is viewed, the two appearances presented would be the same (except for slight differences of the distances of the various

parts). Hence, if we consciously compared the pictures formed on the two retinæ, we should infer the solidity of the object; and this is the impression unconsciously conveyed to the brain. When the two eyes are directed towards one of the objects in the field, there will be formed double images of other objects, that is, images not at corresponding points of the retina. If a finger is held up before the wall, to the two eyes it will be seen to cover two different parts of the wall; or, if the wall be looked at, two indistinct and shadowy images of the finger will be observed before different parts of it. This, however, forms no obstacle to distinct vision: the object which is directly looked at forms a single picture for the brain. The pictures of the other objects are formed on parts of the retina where they are not so distinctly seen; and their being double merely creates the impression of solidity in the whole field of view.

The stereoscope is an apparatus by means of which the combined action of the two eyes in viewing solid objects is well illustrated. Two photographs of an object or group of objects, are taken from slightly different points of view-the positions that the two eyes may occupy in looking at it. These are then viewed, one by each eye, so that each eye

obtains just the same view that it would have of the actual object. The essential part of the stereoscope which enables this to be done properly consists of two prisms of small refracting angle, which are placed before the eyes with the

refracting edge turned inwards, so that each photograph appears somewhat displaced towards the other, and the images of the two, as seen through the two prisms, overlap; also the surfaces of the prisms are slightly convex, so that they also act as convergent lenses, and produce magnified images of the photographs at a greater distance than these from the eyes. The accompanying figures represent the stereoscope and a cross-section of its two prisms.

If the stereoscopic views of a solid geometrical figure, such as a cube, be placed in the apparatus, so that the view for

FIG. 231.

R

the right eye comes before the left, and vice versa, then the impression obtained will be that of a hollow figure,

for each eye then actually gets the view it would have if the two looked directly at a hollow figure. A solid cube standing on a flat surface would appear like a hollow cubical recess let into the surface.

When the normal eye is at rest, that is, when the ciliary muscle is not acting to increase the convergence and diminish the focal length of the crystalline lens, it is focussed for infinite vision, or parallel rays of light coming along the optic axis will be brought to a focus on the yellow spot. And such an eye can with ease bring to a focus on the yellow spot a pencil of light coming from a point at any distance between infinity and about ten inches before the eye. That is, the eye can see distinctly objects at all distances between these limits; that is to say, the images formed are distinct, and not blurred, but they will, of course, if the objects are too far off, be too small for details to be distinguished. When an eye cannot adjust itself for viewing objects at such distances, vision must be regarded as defective. The defects are of two sorts, according as the eye cannot see objects which are far off or those which are close to it.

Some eyes cannot bring a parallel pencil to a focus on the retina. Even when the eye is quite at rest, its focal length is too short, and the focus of such a pencil is formed in front of the retina; or we may say that the retina is too far away from the lens. This defect generally diminishes with advancing age. It is called myopia, or myopy, or, in ordinary language, 'short-sightedness." To correct it, we must notice that the eye can only adapt itself for tolerably divergent pencils; and for viewing a distant object, that is, for receiving pencils

66

of little divergence, a divergent or concave lens must be used. Hence the use of concave spectacles and eye-glasses by the short-sighted. An eye having this defect can generally see distinctly objects much closer to it than the least distance of distinct vision for normal eyes.

Some eyes cannot see distinctly objects which are near. For them the difficulty arises that when an object (such as a printed page) is held far enough off to form a true image on the retina, this image is too small for its details to be recognized. Such an eye is incapable of bringing to a focus on the retina a pencil coming from a point at the ordinary least distance of distinct vision. The focal length cannot be made short enough by the action of the ciliary muscle. This defect generally comes on or increases with advancing age. It is due to a hardening of the crystalline lens; so that the ciliary muscle is unable to give it sufficient curvature and a small enough focal length. It is called presbyopia, presbyopy, or presbytia, or, in ordinary language, "longsightedness." To correct it, notice that the eye cannot adapt itself for a pencil of much divergence; thus these pencils must be made less divergent, and a converging or convex lens must be used. Hence the use of convex spectacles by the long-sighted.

For a person short- or long-sighted, if we know the greatest or the least distance of distinct vision, it is a matter of simple calculation, from the formula for thin lenses in combination, to calculate with sufficient accuracy the focal lengths of the spectacles required-in the first case to produce a combined focal length equal to infinity, and in the second equal to about ten inches, or whatever distances may be required.

EXAMPLES.

1. A screen with a small hole in it is held before the eye, within the least distance of distinct vision, and against a good light. Between this and the eye, and very close to the eye, is held a very small object, such as a pin-head. A large, blurred, and inverted image of the object is seen. Explain this, giving a diagram.

2. The greatest distance at which a short-sighted person can see distinctly is 2 feet. What spectacles does he require so as to see objects at very great distances? and if his least distance of distinct vision without the spectacles was 6 inches, what will it be with them?

3. Find what spectacles must be used by a person who cannot focus his eyes on an object nearer than 2 feet so as to be able to read distinctly at 10 inches' distance.

CHAPTER XXV.

RELATIONS BETWEEN LIGHT AND ELECTRICITY.

We shall now point out some of the relations that exist between magnetic and electric actions on the one hand, and light on the other.

Faraday discovered that if plane polarized light is passed through a transparent substance in a magnetic field, the plane of polarization is, as a rule, rotated. The rotation is greater the smaller the angle between the direction of the rays and the lines of force, and disappears when these two directions are at right angles. This action on light may be shown in the following manner: A strong electro-magnet is furnished with pole-pieces having holes through them in a line with each other so as to allow the light to pass. Between the poles is placed a piece of very dense glass, and two Nicols are placed in the path of a beam of light passing through the two holes and the glass, one on each side of the arrangement. Before making the current in the electro-magnet, the position of the second Nicol is found for which the light coming from the first is quenched. If, then, the current is established so that the glass is in a magnetic field, the light will reappear, and the second Nicol must be rotated to again extinguish it. The sense in which the plane is rotated depends upon the positive direction of the lines of force of the field. reversed, as may be done by reversing the current in the electro-magnet, the sense of rotation is reversed. Thus for light travelling along the lines of force one way the rotation is right-handed, and if it travels in the opposite way the rotation is left-handed. In this way a substance in a magnetic field behaves differently from an ordinary rotating substance such as quartz. From this property it follows that the rotation produced in a beam can be increased by reflecting it to and fro along the lines of force of the field. For suppose the beam is passed along the lines of force, and then reflected back and along again. If the rotation in the first path is right-handed, that in the second is left-handed, and therefore in the same sense in space as the first, and that in the third is again right-handed; so that the entire amount of rotation is three times as much as for a single passage through the medium. If the beam were reflected to and fro through a rotating substance such as quartz, the rotations would cut each

If the field is