52 THEORY OF THE MICROSCOPE. 



bility can be obtained for two definite colours only of the spectrum, 

 for instance, red and violet. For since the dispersion in the 

 crown and flint varies for different pairs of colours, it is not 

 possible, even if the red and violet images exactly coincide, for 

 the images produced by the intermediate rays to coincide with 

 them. The differently-coloured images never appear exactly of 

 the same size ; some exceed the others more or less, and produce 

 the coloured fringe which may always be observed with white 

 light. Hence an approximate elimination of this coloured fringe, 

 sufficient, however, for practical purposes, can only be attained 

 for pencils of rays of a definite inclination. As soon as this 

 inclination is altered, as occurs, for instance, by oblique illumina- 

 tion, chromatic aberration immediately appears. In the full 

 sense of the word, therefore, a double-lens is never achromatic ; 

 and, even supposing that it were to give an absolutely colourless 

 image with axial illumination, it would nevertheless be more 

 or less over-corrected for oblique light. 



The same holds good, of course, for systems of lenses ; it can 

 only be a question of approximating as nearly as possible to 

 achromatism by a skilful combination of flint and crown lenses. 

 In addition to the proper ratio of the focal lengths, the selec- 

 tion of the kinds of glass is important, as those are especially 

 suitable in which the partial dispersions, that is, the relative 

 dispersions of the same pairs of colours (Fraunhofer's lines), differ 

 in the flint and crown as little as possible. 



Moreover, it is obvious that in Microscopes in which the eye- 

 pieces consist of single lenses, and which are consequently not 

 achromatic, the flint lenses of the objective must have a relatively 

 greater dispersive power, as they have to counterbalance not only 

 the crown lenses united to them, but also the eye-piece. The 

 objective must therefore be, what is usually termed, over-corrected. 

 In most cases the influence of the eye-piece is very slight. It 

 must be borne in mind that if the cones of light incident from 

 an object-point are so refracted that the differently-coloured 

 objective-images are displaced laterally, a compensation between 

 the objective and the eye-piece becomes impossible. But this 

 lateral displacement always takes place when the incident cones 

 of light are inclined to the axis on one side, e.g., when the 

 mirror is moved out of the axis. We will discuss this point 

 fully later on (vide On the Testing of the Microscope), in con- 



