56 ELEMENTARY PHOTO-MICROGRAPHY. 



The rough focussing should be done through the 

 eyepiece, with the camera and sliding block re- 

 moved, as explained previously. 



After the camera is replaced, a little time in 

 trying different effects will not be wasted. The 

 effects of closing the iris diaphragm, or a slight 

 oblique light if fine markings are desired, or plain 

 ground-glass between the condenser and the lamp 

 if white diffraction lines are too conspicuous, may 

 all be tried. Perhaps a coloured glass may be 

 better. Then when a suitable result is obtained, 

 one has to be very accurate with the fine adjust- 

 ment, for everything else goes for nothing if correct 

 focus be wanting. 



A vertical camera is better for oil immersion 

 objectives, though there is no difficulty when using 

 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. 



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. 



There are certain slides sold as test objects, and 

 if the photographer wishes to satisfy himself that 



