20 PRACTICAL HISTOLOGY. 



Other methods of preventing spherical aberration are also 

 had recourse to ; thus, in the eyepiece of the microscope, 

 two lenses (E F, Fig. 31) are so arranged that the rays 

 which are internal in passing through the one become ex- 

 ternal in passing through the other, so that the spherical 

 fault of the one corrects that of the other. The manner 

 in which spherical aberration is corrected in the objective 

 would require an explanation entirely beyond the limits 

 of this work. It must, therefore, be simply stated that it 

 is by a combination of converging with diverging lenses, of 

 such curvature and arranged at such distances, that the 

 positive spherical aberration of the one set is neutralised 

 by the negative spherical aberration of the other. 



32. Chromatic aberration is due to the fact that the 

 different coloured rays which compose white light are 

 unequally refrangible ; those of the longest wave length 

 the red being least, and those of shortest wave the violet 



FIG. 23. Chromatic aberration. 



being most easily bent. In passing through a lens, 

 therefore, white light is dispersed, the violet rays are foca- 

 lised (V, Fig. 23) nearer the lens than the red (R), while 

 the foci of the other colours are intermediate between 

 these. The image of a colourless object therefore appears 

 coloured. The coloration is most marked at V and R, 

 least so at F K ; for in that plane there is an intersection 

 of the red and violet rays. 



33. To understand the manner in which chromatic 

 aberration is remedied, it is necessary to carefully distin- 

 guish between the deflection and the dispersion of light. 



