FIG. 28.—Conjugate Foci or Convex Lens. ]
Now let us consider how images are formed. If we take a candle, Fig. 29, and hold the lens a little distance away from it, then, on placing a screen of paper just on the other side of the lens, there will be a small flame depicted on it, an exact representation of the real flame: and it is formed in this way: Consider the rays proceeding from the top of the flame, which are represented separately in Fig. 30, where A represents the top. One of these rays, A _a_, passing through the centre of the lens _o_, will he unaffected because the surfaces through which it passes are parallel to each other; and we know from the property of the lens that all the other rays from A will, on passing through it, be brought to a focus somewhere on A _a_, depending on the curvature of the lens, and in the case of our lens it is at _a_.
FIG. 31.—Dispersion of Rays by a Double Concave Lens. ]
In like manner also all the rays from B are brought to a focus at _b_, and so on with all other parts of A, B, which in this case represents the flame, each will have its corresponding focus; there being cones of rays from every point of the object and to every point of the image, having for their bases the convex lens, and we get an image or exact representation of our candle flame. It will further be noticed that the image _a b_ is smaller than A B, in proportion as the distance _a b_ is less than A B; so that if we increase the focal length of the lens till _a b_ is twice the distance away from the lens, it will become double its present size.
If now the flame be brought nearer the lens, its image _a b_ becomes indistinct; and we must move the screen further away in order to render the image again clear; hence the place of the focus depends on the distance of the object, and the candle and its image must correspond to two conjugate foci.
FIG. 32.—Horizontal Section of the Eyeball. _Scl_, the sclerotic coat; _Cn_, the cornea; _R_, the attachments of the tendons of the recti muscles; _Ch_, the choroid; _Cp_, the ciliary processes; _Cm_, the ciliary muscle; _Ir_, the iris; _Aq_, the aqueous humour; _Cry_, the crystalline lens; _Vi_, the vitreous humour; _Rt_, the retina; _Op_, the optic nerve; _Ml_, the yellow spot. ]