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 



/X. 



.Oeulan 



Draw-tuba, 



_ Racks PCnioa 

 Coarse Adjustment. 



Micrometer Head of 

 Fine Adjustment! 



Stage. 



Ijpper Iris Diaphragm 

 Substage. 



Condenser Mounting. 

 Lower Iris Diaphragm.' 



Condenser Focusing Screw 

 Mirror 



Mirror .. 



Mirror Bar... 



.Fine Adjustment 

 Pillar. 



Horse, Shoe Base 



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 



