CH. 7] 



MICROSCOPE AND ACCESSORIES 



29 



Equivalent 



Focus and 



N. A. of 



Objective 



Diameter 

 of Field 

 in mm. 



Equivalent 



Focus of 



Ocular 



Kind of 

 Ocular 



S5 mm 



15-4 

 io.6 



8.3 



37^4 mm. 

 25' 



I2>2 " 



Huygenian 



45 mm 



7.0 



4.0 



37^2 mm. 

 25 

 123-2 " 



Hu3'genian 



17 mm. 



N. A. = 0.25 



3-° 

 2.0 

 1.6 



5-7 



2.S 



1.4 

 0.97 



37 yi mm. 



25 / " 

 1 2 } 2 " 



Huygenian 



1S0 mm. 

 45 

 15 

 10 ", 



Compensation 



5 mm 



N. A. = 0.92 



0.541 

 0.371 

 0.290 



37^4 mm. 



25 



1 2 % " 



Huygenian 



0.S50 

 0.501 

 0.250 

 Q-I73 



1 So mm. 

 45 

 15 

 10 



Compensation 



2 mm. . 



N. A. = 1.25 



1 



0.270 

 0.186 

 0.147 



37 % mm. 



25 



12% " 



Huygenian 



0.450 

 0.251 

 0.125 

 0.0SS 



I So mm. 

 45 

 15 

 10 



Compensation 



FUNCTION OF AN OBJECTIVE 



S S3- Put a 50 mm. objective on the microscope or screw off the 

 front combination of a 16 mm.,( 2 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. 



