THE OBJECTIVE 121 



If we regard the highest useful magnification as 1, ()()() 

 times the working aperture (which allows 4 times the ordi- 

 nary limit of good eyesight in just seeing details), then the 

 highest allowable eyepiece magnification is this figure 

 divided by the magnification number of the objective. 

 This presupposes a faultless condenser cone nearly equaling 

 the objective in aperture. Thus for the apochromatic 

 objective 10, of 0.3 aperture, the highest useful magnifica- 

 tion would be 300, and the highest allowable eyepiece 

 magnification would be 30. For the 60 and 90 objectives 

 of aperture 1.3, the highest useful eyepiece magnifications 

 would be 22 and 14, respectively. Comparing the 

 objectives of 1.3 and 1.4 aperture and 90 magnification 

 (2-millimeter focus), we have 14.4 and 15.6 for highest eye- 

 piece magnifications, and these are not markedly different. 

 In practice the eyepiece magnifying 15 times would be 

 used for both. 



The table gives the limit for optimum results with 

 yellow-green light, a perfect condenser, and maximum 

 contrast in the object. The figures are mainly calculated 

 from optical theory, not derived wholly from experiment 

 (this being difficult). 



Adjusting the Objective. — The adjustment of the objec- 

 tive by the user is for spherical aberration only. An uncor- 

 rected lens has a shorter focus for its marginal zone, the 

 focus decreasing from the center to the margin. An 

 undercorrected lens has more or less of this state left. 

 An overcorrected lens has shorter focus in the center than 

 at the margin. A corrected objective can only be in adjust- 

 ment for one definite working distance, which depends on 

 the circumstances given below. 



Causes of Overcorrection Causes of Undercorrection 



1 . Denser mounting medium than Mounting medium of lower refrac- 



air, water, or cedar oil, respec- tion than that before the objective, 

 tively. 



2. Too thick cover-glass. Too thin cover-glass. 



3. Denser immersion medium than Immersion medium of less refraction 



water, or cedar oil, respectively. than the objective was calculated 



for. 



