Objectives. 48 



Numerical aperture 1*0 representing the air angle of 

 180, and the equivalents in water and oil above given. 



To enable the numerical apertures of objectives to be 

 taken without a calculation, Professor Abbe introduced the 

 apertometer (see Fig. 7). This consists of a semicircular 

 plate of crown glass, with the diametrical edge ground to 

 an angle of 45. The centre of the semicircle has a silver 

 disc with a very small opening; two pointers are fitted 

 on the outer edge of the instrument, and a scale is marked 

 on the upper surface. This is laid on the stage of the 

 microscope, and the objective screwed on that it is proposed 

 to test. A low-power object-glass is included with the 

 instrument, and must be fitted into the lower end of the 



E:^* r 



Carl Zeiss Apertometer ^ 



* 



FIG. 1. Abbe Apertometer. 



drawtube of the microscope, the eyepiece being inserted as 

 usual. The edge of the apertometer is then illuminated 

 by means of a bull's-eye condenser and lamp, and the two 

 pointers set at the centre of the outer circle. The drawtube 

 must now be moved carefully up and down until these 

 pointers are seen sharply in the centre of the field. Each 

 is then moved round the outer circle of the apertometer in 

 opposite directions until the edge of the field is reached. 

 The reading is then taken. In the case of an oil or water 

 immersion lens, the medium must, of course, be placed in 

 front of the object-glass. 



It has been explained by Dr. Dallinger, in ' Carpenter on 



