THE OPTICAL EQUIPMENT 



39 



power, and are called by the letters A, B, C, D, E and F, the 

 magnification usually being 6, 9, 12, 18, 21, and 24. 



The images formed by achromatic objectives are in them- 



selves complete, and the ocular should cause no subsequent 

 alteration in their optical quality. In the case of the 

 apochromatic objectives this is not so. When these lenses 



