The effect of a convex lens is exhibited in Fig. 178. It brings rays of light passing through It to a point or focus (F) by refracting or bending them out of their course. Now sup pose that the object from which the light is reflected is an arrow, as in Fig. 179. The rays of light from the point of the arrow (A) are acted on by the lens and brought to a focus at a, the rays from the other end of the arrow are focussed by the lens at b, and rays from every other point of the candle are focussed at corre sponding points between a and b, so that rays from every point of A n have corresponding points in the line a b. In short, an image of A B is produced at a b through the agency of the convex lens, but the image is upside down, because, as we see, a is the image of A, and b of 13. Now if at a b a screen were placed, and if all light except that passing through the lens were prevented falling on the screen, a bright distinct image of the arrow would be seen, but smaller than the real object and upside down. The conditions would be accurately fulfilled if the screen were on the back wall of a black box which had an opening in front in which was fixed the lens. But this is just a camera with its ground-glass plate as screen. The inside of the box is blackened to prevent reflection of light which would mar the distinctness; of the image. Now from Fig. 179 it will be evident that if A B were brought nearer to the lens, its image a b would not be found in the same place. It would be further removed. The screen would require to be moved back a bit. Suppose the screen were immovable, the lens might he altered in position so as to bring the focus once more on to the screen. If the lens could not be moved, nor the screen, the new position of AB would cause its image to fall behind the screen. If another lens were placed in front or behind the original one its action would be strengthened, the rays would be brought to a focus sooner, and if the added lens had the proper degree of convexity (of thickness in the middle) the image would be brought forward so as to make it once more fall on the screen. Now in a pher's camera the screw which moves the lenses backwards or forwards in their outer tube is for the purpose of bringing the focus always on the ground-glass plate. Usually also the box is made so that it can be lengthened or shortened to effect the same purpose, for the lenses always remain the same. Another thing remains to be noticed about the photographer's camera. Lenses focus more quickly rays of light passing through them near the circumference than those passing through the centre. Both sets of rays are not focussed at the same point. If the rays come from an object, the image produced is not defi nite, because all the rays of light are not equally focussed. Now in a camera this is corrected by the use of a stop or diaphragm. It consists of a plate of metal with a hole in the centre. This is passed through a alit in the metal tube between the lenses. It cuts off the outside rays, the centre ones only passing. The holes are made of various sizes to suit the amount of light. By means of the stop an element of con fusion is removed and the image made very distinct.
Now if this description of a photographer's camera be applied to Fig. 174 it will be evident how accurately it represents the purposes of the eyeball. The eyeball is a chamber with com pact walls into which light can pass only through a clear portion in front (the cornea). Like the camera the eyeball has a dark coat to prevent reflection of light, the dark cho roid. Towards the front is a lens—the crystal line lens—through which all rays of light that enter the eye must pass. The lens focusses the rays as any ordinary lens would do. But the action of the lens is aided. There are several refractive substances forming the eye. The cornea refracts slightly, so also does the aqueous humour filling the anterior chamber, and the vitreous humour filling the posterior chamber does so to a greater extent than either cornea or aqueous humour. Thus the moment rays of light enter the eye they begin to be bent out of their course, and the result of the action of the lens, aided by the cornea and aqueous and vitreous humours, is that rays of light that are parallel when they fall upon the eye are brought to a focus on the back wall. If then an object is a
long distance off, rays of light proceeding from it and falling on the eye are brought to a focus on the back wall of the eye, and there will be produced a small image of the object upside down (Fig. 180). Suppose the object is brought near, then, as we have seen with a lens, the image would fall beyond the wall of the eyeball. To secure that it fall on the wall exactly, one of three things is necessary, as we have seen, the wall must be moved further back, or the lens must be capable of movement, or there must be some way of increasing the focussing power of the lens, so that the rays are sootier brought to a focus, and thus made, once more, to fall on the wall. In the eye it is the convexity of the lens that is altered, and by this means the eye is capable of accommodating itself to different distances, as it is phrased.
Accommodation of the Eye to different distances. We are continually moving our eyes from object to object, now looking at something at a distance, now at something, near, and again at something far off. To see each thing tinctly the eye must be capable of altering itself with great precision and rapidity to suit the varying distances. The lens is a very elastic body, as stated on p. 448, and is confined within a capsule which presses upon it, and flattens it somewhat. But the pressure of the capsule may be relaxed by contraction of the ciliary muscle (p.448), so that the lens bulges forwards and becomes more convex. When we look at a near object the ciliary muscle contracts, the capsule relaxes, the lens bulges forwards, the rays of light are thereby more refracted and the image of the object is distinctly produced on the back of the eyeball. When the object is nearer, the ciliary muscle contracts more, and the lens becomes still more convex. When the object is far away, if the lens were to remain as before, the image would be formed in front of the back of the eyeball, and, therefore, the ciliary muscle relaxes, the capsule tightens, the lens is flat tened slightly, refracts less strongly, and the image is formed on the back wall as before.
Normal or regular Sight exists when the degree of convexity of the lens and the length of the eye are such that rays of light coming from a distance are brought to a focus on the retina—the lining of the back of the eye—without any effort of the eye, the eye remaining at rest. Practically all objects at a distance of about 70 yards and upwards from the eye require no effort of dation. This distance from the eye is the far point at which the need of accommodation; ceases, and it has been called the punctum remotum, or far point. As soon as an object comes nearer than that, the lens must begin to become more convex, and the nearer the object comes the more does the lens increase in con vexity by the contraction of the ciliary muscle, till the object is so near that every effort is made to produce greater contraction and thereby greater convexity, and a sense of straining is experienced. A point is at last reached so near to the eye that no further accommodation can be effected, and, if the object is brought nearer, it is no longer distinctly seen. This point is the punctum proximum or near point, and for the ordinary eye the distance is about six inches. In other words, from an object, distant 75 yards and upwards from the eye, reflected rays of light falling upon the eye, and passing through its lens, humours, &c., conic naturally to a focus on the retina and form an image there, without any effort of the eye. Rays from au object any nearer than that would be focussed behind the eye were no effort made, but by the arrange ments for accommodation the lens becomes more convex and the rays are focussed sooner, so that they again fall on the retina. As the body comes nearer and nearer the effort on the part of the eye to focus the rays becomes greater and greater till no greater effort can he put forth, and if the body be nearer than six inches the effort is not sufficient, the lens cannot be come convex enough, and thus the rays are no longer brought to a focus on the retina, and in consequence the body can no longer be dis tinctly seen.