28 



MICROSCOPE AND ACCESSORIES. 



\_CH. I. 



The size of the field is also dependent, in part, without regard to 

 magnification, upon the size of the opening in the ocular diaphragm. 

 Some oculars, as the orthoscopic and periscopic, are so constructed as 

 to eliminate the ocular diaphragm, and in consequence, although this 

 is not the sole cause, the field is considerably increased. The exact 

 size of the field may be read off directly by putting a stage micrometer 

 under the microscope and noting the number of spaces required to 

 measure the diameter of the light circle. 



§ 47. Table showing the actual size in millimeters of the field of a group 

 of commonly used objectives and oculars. Compare zcil/i the graphic rep- 

 resentation in Fig. j>7. See also § 4.6. 



Equivalent 



Focus and 



N. A. of 



Objective. 



Diameter 

 of Field 

 in mm. 



Ocular. 



85 mm. . . . 



15-4 

 10.6 



8-3 



37^ 



25 



12^ 



mm. 



Huygenian. 



45 mm. . . . 



7.0 

 4.0 



37^ 



25 



12^ 



mm. 



Huygenian. 



N.A. =0.25 



3-o 

 2 

 1.6 



25 



12J-2 



mm. 



Huygenian. 



5-7 

 2.8 



1-4 

 0.97 



180 

 45 

 15 

 10 



mm. 



Compensation. 



5 mm. . . . 

 N.A. =0.92 



54* 

 0371 

 0.290 



11%. 



25 



12K 



mm. 



Huygenian. 



0850 

 0.501 

 250 

 0.173 



180 

 45 

 15 

 10 



mm. 



Compensation. 



i 



2 mm. . . . 

 N.A. = 1.25 



0.270 

 0.186 

 0.147 



37 % 

 25 



12-4 



t< 



Huygenian. 



0.450 

 0.251 

 0.125 

 0.088 



l8o 



45 

 15 

 10 



mm. 



Compensation. 



