A A′ represent the parallel rays of white light falling on the lens, L L′. The blue rays are bent more than the red, so the principal focus of the former is at C and of the latter at D. The consequence of this difference in bending of the various coloured light rays would be most serious in microscopic work were not means devised to overcome it. Objects for instance at C in our diagram would appear blue, at D they would appear red, whilst at E E′ though no single colour would predominate they would be illumined with many coloured rays, though less strongly than at C or D. This chromatic aberration, as it is called, depends amongst other things on the nature of the glass used in lens construction. It has been found, however, that a combination of flint and of crown glass will overcome the difficulty. In a later chapter we shall explain the difference between these two kinds of glass. In practice, a plano-concave lens of flint glass is combined with a double convex lens of crown glass and, if the nature of the glass is satisfactory, as also the shapes of the lenses, there is full correction for chromatic aberration, and objects viewed through such a lens will not appear with coloured margins. There is one further trouble likely to occur in such, or any lens. We write of the rays meeting at a point. In our diagrams we represent the rays by straight lines, really they are much more complicated than they appear in a diagram. It is quite easy to take a ruler and make our imaginary light rays meet at a point, as a matter of fact, where real lenses and real light rays are concerned, it is very difficult, if not impossible, to make the latter meet at a single point. One more diagram may make the matter clear. [Illustration] Parallel rays A pass through our lens and, as we know, they should all meet at a point P, the principal focus of the lens; the majority do so, but some meet at other points, such as P′. In consequence of this it is difficult to obtain a clear image of an object at P, and the lens is said to suffer from spherical aberration. The perfect simple lens would be one fully corrected for chromatic and spherical aberration. CHAPTER IV THE COMPOUND MICROSCOPE