112 



perience has taught them are most generally 

 useful. 



It is safe to follow in all work on recognized 

 forms (objects of which the structure is known) 

 the rule, not to use a higher power than is necessary 

 to properly study them. 



Apochromatic Objectives. All objectives of 

 what may be termed the ordinary corrections have 

 a residual chromatic and spherical aberration, the 

 former being called the secondary spectrum, but 

 Prof. Abbe has in this direction also effected a 

 notable improvement, which with a uniform cor- 

 rection of the spherical aberration, corrects for 

 three colors, thus resulting in a closer concentration 

 of image-forming rays which, with the greater 

 numerical aperture made possible by these con- 

 ditions, results in a higher resolving power. It 

 has been found however, that in high powers of 

 wide aperture, even with these objectives, there is 

 an outstanding error which by itself cannot be cor- 

 rected. This is balanced in the eyepiece, which is 

 correspondingly over-corrected and is called the 

 .compensating eyepiece. It has therefore been 

 necessary to make the low powers under-corrected 

 as well, so that they might be used with the same 

 eyepieces, for it is evident that neither these 

 objectives with the Huyghenian eyepiece, nor the 

 compensating eypiece with the achromatic objec- 

 tives will give satisfactory results. 



