298 FINDING THE NUMERICAL APERTURE [Ch. IX 



of Tolles, but it is carried out in a simpler manner; it is shown in 

 fig. 177. As seen by this figure it consists of a semicircular plate of 

 glass. Along the straight edge or chord the glass is beveled at 45°, 

 and near this straight edge is a small, perforated circle, the perfora- 

 tion being in the center of the circle. To use the apertometer the 

 microscope is placed in a vertical position, and the perforated circle 

 is put under the microscope and accurately focused. The circular 

 edge of the apertometer is turned toward a window or plenty of 

 artificial light so that the whole edge is lighted. When the objective 



Fig. 177. Abbe Apertometer. 



is carefully focused on the perforated circle the draw-tube is removed 

 and in its lower end is inserted the special objective which accom- 

 panies the apertometer. This objective and the ocular form a low 

 power compound microscope, and with it the back lens of the objective, 

 whose aperture is to be measured, is observed. The draw-tube is 

 inserted and lowered until the back lens of the objective is in focus, 

 " In the image of the back lens will be seen stretched across, as it were, 

 the image of the circular part of the apertometer. It will appear as a 

 bright band, because the light which enters normally at the surface is 

 reflected by the bevel part of the chord in a vertical direction so that 

 in reality a fan of 180° in air is formed. There are two sUding 

 screens seen on either side of the apertometer; they slide on the verti- 

 cal circular portion of the instrument. The images of these screens 

 can be seen in the image of the bright band. These screens should now 

 be moved so that their edges just touch the periphery of the hack lens. 

 They act, as it were, as a diaphragm to cut the fan and reduce it, so 

 that its angle just equals the aperture of the objective and no more." 



