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. [Illustration: Fig. 19.--Correction of Chromatic Aberration.] The achromatic aplanatic objective constructed on the optical formula enunciated, did not meet all the difficulties experienced by the skilled microscopist, in obtaining resolution of the finest test objects, and whereby the intrinsic value of the objective (in his estimation) must stand or fall. There were other disturbing residuary elements besides those of the secondary spectrum, and which at a later period were met by the practical skill of the optician, who applied the screw-collar, and by means of which the back lens of the objective is made to approach the front lens, thus more accurately shortening the distance between the eye-piece, where the image is eventually formed, and the back lens of the objective. In this diagram L L is a _convex_ lens of _crown-glass_, and _l l_ a _concave_ one of _flint-glass_. A convex lens will refract a ray of light (S) falling at F on it exactly in the same manner as the prism A B C, whose faces touch the two surfaces of the lens at the points where the ray enters, and quits. The ray S F, thus refracted by the lens L L, or prism A B C, would have formed a spectrum (P T) on a screen or wall, had there been no other lens. [Illustration: Fig. 20.--Virtual Image formed by Convex Lens.]