Page:Optics.djvu/114

 of the lens for that of the reflecting surface; the image is as there a portion of a conic section, the latus rectum of which is the principal focal length of the lens. The curvature of the image is the same for all distances.

The reader will have no difficulty in collecting from this that the image of the Sun given by a lens is a small portion of a spherical surface, the middle of which is at the principal focus of the lens.

It will moreover be easily seen (Fig. 134 and 135.) that a double-concave lens gives an erect image, a double-convex generally an inverted one. I say generally, because when the object is nearer the lens than the principal focus, (Fig. 136.) the image is beyond it on the same side, and therefore erect.

With the exception of this last case, the image given by a convex-lens is a real one, that by a concave-lens is always imaginary.

Note. In all images produced by reflexion or refraction at spherical surfaces, there is in practice necessarily some little indistinctness arising from the aberration of the rays distant from the axis. In consequence of this, instead of single points or foci we find small circular spots with bright edges and centres which intersect and confuse each other as represented on an exaggerated scale in Fig. 137.

122. come now to an entirely new part of the subject. We have hitherto considered light as something, whether substance or affection of substance, of one single kind. The fact is, however, that light, as it comes to us from the Sun, is not homogeneous, but consists of various kinds, mixed together in different proportions, which being refrangible in different degrees by the same medium, may be exhibited very clearly by causing a Sunbeam to pass through a prism and fall on a sheet of paper placed in a proper position or the wall of a room. The parcels of different kinds of rays being unequally refracted are separated, and the result is a lengthened spectrum, such as is shown in Fig. 138, the colour of which is at one end deep red, and passes through various gradations of orange, yellow, green, and blue to a reddish violet. It appears