THE OPTICAL EQUIPMENT 55 



of the total aperture of the objective; the illuminating 

 area, therefore, consists of an annular zone amounting to 

 approximately the remaining one-third of the aperture. In 

 addition, any aperture that the condenser may have in excess 

 of the objective aperture may be utilised. 



To obtain the maximum illumination and the best result, 

 the size of the central stop must be exactly suited to the 

 objective. Most makers supply a series of these with their 

 sub- stage condensers, but Messrs. Zeiss now have them marked 

 to suit any particular objective. The consequence is that 

 dark-ground illumination may be obtained with a condenser of 

 either 1-20 or 1-40 N.A., and the stop that should be used with 

 any particular lens is clearly marked as such. 



In the latter case, the stop is supported in the condenser by 

 a star disc, which is inserted into the diaphragm carrier, and 

 which has a central button pointing upwards. Each stop has a 

 small central hole which drops over the button. It is important 

 to see that the condenser is exactly centred, as on this depends 

 the accuracy of the result. The iris- diaphragm should be fully 

 open except when using low-power objectives, with which a 

 better result is often obtained by partially closing it. This is 

 however only a matter of ordinary observation ; the best effect 

 is easily seen and the appliance can be regulated accordingly. 



The best condenser for the purpose is an oil-immersion having 

 a N.A. of 1'40, and with this arrangement objectives as high 

 as a one-sixth inch may be satisfactorily used. The source of 

 light must in this case be so projected that if the plain surface 

 of the mirror on the microscope is used, it is illuminated with 

 a broad beam of light ; or, in the case where the microscope is in 

 a horizontal position and the mirror therefore thrown out, that 

 the back lens of the condenser should be completely filled with 

 light. This is of the highest importance, as it is obviously on the 

 use of the marginal portions of the condenser that the illumina- 

 tion depends, and this part therefore must be completely 

 and evenly filled. It will be found that it is necessary to 

 bring the condenser close up to the undersurface of the 

 object-slide; and to obtain the best results the thickness of the 

 object-slide must be exactly suited to the condenser. It is 

 important to see that the central stop is strictly at the optical 

 centre of the condenser, since any want of centration will largely 



