68 ELEMENTARY PHOTO-MICROGRAPHY. 



them in the horizontal position, if only just suffi- 

 cient oil be added to make the connection between 

 coverslip and objective. A drop of oil will remain 

 for hours without running or spreading, provided 

 it be carefully and not too profusely applied. 



FINE FOCUSSING WITH HIGH POWERS. 



In focussing with high powers, the danger of 

 cracking the cover glass is lessened if the mounted 

 slip be raised from the stage at one edge, using 

 a finger-nail for this purpose, and roughly following 

 up the slip until it is flat on the stage again, when 

 the fine adjustment comes into play. 



The finest ground-glass screen is much too 

 coarse to show minute details even when viewed 

 through a focussing glass, and with high powers 

 the projected image on the screen as a rule is 

 anything but brilliant. Some other method must, 

 therefore, be adopted for focussing an object when 

 using, say, such a power as ^ oil immersion lens. 

 Perhaps the best way is to dispense with the 

 ground-glass screen altogether, and focus from the 

 aerial image caught by a hand reading glass or the 

 Bousfield spectacle lens. 



A little practice is certainly essential before 

 the object can be seen at all ; but after the knack 

 of catching the aerial image is once mastered, 

 focussing becomes easy. 



