Apertures of Objectives Angular and Numerical 6B 



power, that is supplied with the apparatus, is screwed into 

 the universal thread, which should be there fitted in all 

 microscopes of high-class. 



The cylindrical edge of the apertometer is then illumi- 

 nated in front and at the sides by means of bull's-eye 

 condensers and lamps, or, if daylight is available, it will 

 be easier to get uniform brilliance all over the field by 

 placing the microscope on a table in front of a window, 

 and using bull's-eye condensers to increase the light. 



The draw-tube carrying the special objective and the eye- 

 piece is then replaced in the body of the microscope and 

 the image of the pointers b in Fig. 17 is brought sharply into 



FIG. 17. ABBE'S APERTOMETEK. 



focus in the centre of the field by slowly extending the draw- 

 tube, being reflected by means of the prismatic diametrical 

 edge of the glass plate. While looking through the micro- 

 scope these pointers are then each moved separately, in 

 opposite directions, round the outer edge of the apertometer,. 

 until they are set exactly on the extreme margins of the 

 back lens. The reading is then taken by the divisions on 

 the face of the apertometer against which the pointers have 

 now arrived. In the case of an oil or water immersion 

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

 object-glass during the examination. 



There are several further matters of importance that it 

 would be well to consider. 



