A little reflection will show that in consequence of this unequal refrangibility of the coloured rays, white light must invariably be decomposed on entering any dense medium. This is indeed the case; but if the surfaces of the medium _are parallel to each other_ the effect is not seen, because the rays recombine on their emergence, being bent to the same extent in the opposite direction. Hence light is transmitted colourless through an ordinary pane of glass, but yields the tints of the Spectrum in its passage through a prism or a lens, where the two surfaces are inclined to each other at an acute angle. Chromatic aberration is corrected by combining two lenses cut from varieties of glass which differ in their power of separating the coloured rays. These are the dense flint-glass containing Oxide of Lead, and the light crown-glass. Of the two lenses, the one is _biconvex_, and the other _biconcave_; so that when fitted together they produce a compound Achromatic lens of a meniscus form, thus:-- [Illustration] The first Lens in this figure is the flint- and the second the crown-glass. Of the two the biconvex is the most powerful, so as to overcome the other, and produce a total of refraction to the required extent. Each of the Lenses produces a spectrum of a different length; and the effect of passing the rays through both, is, by overlapping the coloured spaces, to unite the complementary tints, and to form again white light. Spherical Aberration of Lenses.--The field of a Camera is not often equally sharp and distinct at every part. If the centre be rendered clear and well defined, the outside is misty; whilst, by slightly altering the position of the ground glass, so as to define the outside portion sharply, the centre is thrown out of focus. Opticians express this by saying that there is a want of proper flatness of field; two causes may be mentioned as concurring to produce it. The first is "spherical aberration," by which is meant the property possessed by Lenses which are segments of spheres, of refracting rays of light unequally at different parts of their surfaces. The following diagram shows this:-- [Illustration] Observe that the dotted lines which fall upon the circumference of the Lens are brought to a focus at a point nearer to the Lens than those passing through the centre; in other words, the outside of the Lens refracts light the most powerfully. This causes a degree of confusion and indistinctness in the image, from various rays crossing, and interfering with, each other. Spherical aberration may be avoided by increasing the convexity of the centre part of the Lens, so as to add to its refracting power at that particular point. The surface is then no longer a segment of a sphere, but of an ellipse, and refracts light more equally. The difficulty of grinding Lenses to an elliptical form however is so great, that the spherical Lens is still used, the aberration being corrected in other ways.