CH. /] 



MICROSCOPE AND ACCESSORIES 



29 



I Equivalent 

 Focus and 

 N. A. of 

 Objective 



Diameter 

 of Field 

 in mm. 



Equivalent 



Focus of 



Ocular 



Kind of 

 Ocular 



85 mm 



15-4 

 10.6 



3-3 



37K mm. 

 25 

 12K " 



Huygenian 



45 mm 



7.0 



5-o 

 4.0 



37^ mm. 

 25 



I2>< " 



Huygenian 



17 mm. . 

 N. A. =0.25 



3-0 

 2.0 

 1.6 



37^ mm. 

 25 

 12K " 



Huygenian 



5-7 



2.8 



1.4 

 o.97 



180 mm. 

 45 

 15 

 10 " 



Compensation 



5 mm. . .. 

 N. A. =0.92 



o.54i 

 0.371 

 0.290 



37^ mm. 

 25 

 1234 " 



Huygenian 



0.S50 

 0.501 

 0.250 

 O.I73 



180 mm. 

 45 

 15 

 10 " 



Compensation 



2 mm. . 



N. A. = 1.25 

 



0.270 

 0.186 

 0.147 



37K mm. 

 25 

 12^ " 



Huygenian 



0.450 

 0.251 

 0.125 

 0.088 



180 mm. 

 45 

 15 

 10 " 



Compensation 



FUNCTION OF AN OBJECTIVE 



§ 53. Put a 50 mm. objective on the microscope or screw off the 

 front combination of a 16 mm.,(^3-in.), and put the back combination 

 on the microscope for a low objective. 



Place some printed letters or figures under the microscope, and 

 light well. In place of an ocular put a screen of ground glass, or a 

 piece of lens paper, over the upper end of the tube of the microscope.* 



*Ground glass may be very easily prepared by placing some fine emery or 

 carborundum between two pieces of glass, wetting it with water and then rubbing 

 the glasses together for a few minutes. If the glass becomes too opaque, it may 

 be rendered more translucent by rubbing some oil upon it. 



