44 



ELEMENTARY CHEMICAL MICROSCOPY 



tive mounting. In the case of Fig. 18, the objective is also 

 unscrewed just above the back lens combination, but in this 

 case the diaphragm is merely dropped into the hole in the lower 

 half of the mounting, while in the case shown in Fig. 19, the 

 long tubular diaphragm is inserted into the objective from above 



FIG. 17. FIG. 18. FIG. 19. 



Methods of Reducing Numerical Aperture of Objectives for Dark-ground Studies. 

 (D, D, D, Removable Diaphragms.) 



without necessitating any separation in the mounting of the 

 objective lenses. By means of these diaphragms the numerical 

 apertures of the objectives are reduced to approximately 0.9 

 or 0.95. 



In order to obtain the maximum resolving power with dark- 

 ground illumination Conrady has shown 1 that the condenser 

 must have not less than three times the numerical aperture of 

 the objective. He suggests that the practical resolving power 

 obtainable may be expressed as equal to J N.A. objective + 

 i N.A. condenser, but Reinberger points out that on actual 

 trial 2 the Conrady formula gives results about 25 per cent too 

 low. The inexperienced observer, however, will find that the 

 resolving power obtainable in his work will conform rather 

 closely with the Conrady formula. It is therefore well to bear 

 in mind that in dark-ground illumination studies fine details of 

 structure are to be discerned only with the greatest difficulty 



1 Conrady, J. Quekett Micro. Club, 11 (1912), 475. 



2 Reinberger, J. Quekett Micro. Club, 11 (1912), 503. 



