If parallel rays of light pass through a double-convex lens the violet rays, the most refrangible of them, will come to a focus at a point much nearer to the lens than the focus of the red rays, which are the least refrangible; and the intermediate rays of the spectrum will be focussed at points between the red and the violet. A screen held at either of these foci will show an image with prismatic fringes. The white light, A A′′ (Fig. 18), falling on the marginal portion of the lens is so far decomposed that the violet rays are brought to a focus at C, and crossing there, diverge again and pass on to F F′, while the red rays, B B′′, do not come to a focus until they reach the point D, and cross the divergent violet rays, E E′. The foci of the intermediary rays of the spectrum (red, green, and blue) are intermediate between these extremes. The distance, C D, limiting the blue or violet, and the red is termed the longitudinal chromatic aberration of the lens. If the image be received upon a screen placed at C, violet will predominate and appear surrounded by a prismatic fringe, in which violet will predominate. If the screen be now shifted to D, the image will have a predominant red tint, surrounded by a series of coloured fringes in an inverted order to those seen in the former experiment. The line E E′ joins the points of intersection between the violet and red rays, and this marks the mean focus, the point where the coloured rays will be least apparent.
[Illustration: Fig. 18.--Chromatic Aberration of Lens.]
In the early part of this century the optical correction of chromatic aberration was partially brought about by combining a convex lens of crown-glass with a concave lens of flint-glass, in the proportion of which these two kinds of glass respectively refract and disperse rays of light; so that the one medium may by equal and contrary dispersion neutralise the dispersion caused by the other, without at the same time wholly neutralising its refraction. It is a curious fact that the media found most available for the purpose should be a combination of crown and flint-glass, of _crown-glass_ whose index of refraction is 1·519, and dispersive power 0·036, and of _flint-glass_ whose index of refraction is 1·589, and dispersive power 0·0393. The focal length of the convex crown-glass lens must be 4-1/3 inches, and that of the concave flint-glass lens 7-2/3 inches, and the combined focal length 10 inches. The diagram (Fig. 19) shows how rays of light are brought to a focus, nearly free from colour. The small amount of residual colour in such a combination is termed the _secondary spectrum_; the violet ray F Y, crossing the axis of the lens at V, and going to the upper end P of the spectrum, the red ray F B going to the lower end T. But as the flint-glass lens _l l_, on the prism A _a_ C, which receives the rays F V, F R, at the same points, is interposed, these rays will unite at _f_, and form a small circle of white light, the ray S F being now refracted without colour from its primitive direction S F Y into the direction F _f_. In like manner, the corresponding ray S F′ will be refracted to _f_, and a white colourless image be the result.