Stargazing: Past and Present

by Sir Norman Lockyer

Fig. 30.—Diagram explaining Fig. [29].

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.

If now rays be passed from the lantern or sun through a concave lens, Fig. [31], they are not brought to a focus, but are dispersed and travel onwards, as if they came from a point, F, which is called its virtual focus; and if rays are first converged by a convex lens, and then, before they reach the focus are allowed to fall on a concave one, we can, by placing the lenses a certain distance apart, render the converging rays again parallel; or we can make them slightly divergent, as if they came, not from an infinite distance, but from a point a foot or two off. The application of this arrangement will appear hereafter.

What has now been said on the action of the convex lens will enable us to consider the optical action of the eye, without which we do little in astronomy. As to the way that the brain receives impressions from the eye we need say nothing, for that belongs to the domain of physiology, except indeed this, that an image is formed on the retina by a chemical decomposition, brought about by the dissociating action of certain rays of light in exactly the same way as on a photographic plate. Optically considered, the eye consists of nothing more than a convex lens, Cry, Fig. [32], and a surface, Rt, extending over the back of the eyeball, called the retina, on which the objects are focussed, but the rays of light falling on the cornea Cn, are refracted somewhat, so that it is not quite true to say that the crystalline lens does all the work, but for our present purpose it is sufficiently correct, and we shall consider their combined action as that of a single lens.