In both objectives and eyepieces the lenses decrease 

 in size with the increase in power and consequently 

 gather less light and while this one objection exists in 

 the objective, an additional one occurs in the eyepiece, 

 in that the eye must be brought closer to the eye-lens 

 and must be kept more strictly in the optical axis, 

 which at a long sitting, becomes fatiguing. 



Choice of eyepiece should be determined by require- 

 ments and individual preference, but the use of high 

 power eyepieces should be avoided whenever possible. 

 All responsible makers of microscopes make up such 

 outfits of stands, objectives, and eyepieces as experience 

 has taught are most efficient and convenient. 



In all work on recognized forms (objects of which 

 the structure is known) it is safe to follow the rule, not 

 to use a higher power than is necessary to properly 

 study them. 



Apochromatic Objectives. All achromatic 

 objectives have a residual chromatic and spherical 

 aberration, the former being the secondary spectrum : 

 but Prof. Abbe has in this direction also effected a not- 

 able improvement, which with a uniform correction 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 conditions results in a higher resolv- 

 ing power. It has been found, however, that, in high 



