Appendix A : The Microscope and Its Optical Principles 139 



1. Cutting off the marginal rays. 



2. Changing the shape of the surface of the lens. 



3. Combining several lenses equivalent to a single lens. 

 Chromatic Aberration. As with a prism, ordinary light in pass- 

 ing through a lens is broken up into its component colors. This 



.Oeulan 



, --w^vori ITII.OTV 



Coarse Adjustment: 



Condenser Mounting.. 

 LtKer Iris Diaphragm. 1 



-Horse. Shoe Dase. 



FIG. 51. A Compound Microscope with Parts Named. 



process is technically termed dispersion. Since the colors are 

 not all bent to the same extent, the result is that each color has a 

 different focus; the ones which are bent most (violet rays) come 

 to a focus nearest the lens and those which are least affected (red 



