ELEMENTARY PHOTO-MICROGRAPHY. 35 



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 adjustment, for every- 

 thing else goes fo>r nothing if correct focus be 

 wanting. 



A vertical camera is better for oil immersion ob- 

 jectives, 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 

 cover slip 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 

 his objectives possess power of penetration, defini- 

 tion, flatness of field, and coincidence of visual and 

 actinic foci, he cannot do better than test the value 

 of his lenses by taking a photograph of some well- 

 known object suitable for the power used. Any 



