92 
PRACTICAL PHOTO-MICROGRAPHY. 
proper position on the ground-glass. I he rough focus 
may be adjusted by holding a piece of white card in one 
hand, projecting the image on it, and working the coarse 
adjustment of the microscope with the other. The 
plain glass is now put in the place of the ground-glass, 
and the image is focussed with a Ramsden or Aplanat 
ocular, the ocular previously focussed on the front of 
the plain glass, and resting lightly on the back of it. 
By means of the fine adjustment rod, or pulley, the 
focus is accurately adjusted, the glass removed, and 
the dark slide, containing the sensitive plate, put in its 
place. The flap shutter of the camera is closed, the 
dark slide opened, and the exposure made. After 
exposure the slide is closed, withdrawn, and taken to 
the operating room for development. 
The above operations represent generally all that are 
necessary with whatever objectives we work, but with 
high powers greater care is needed in matters of accuracy 
of centring, focussing, etc. When we are dealing with 
objectives which require correction by collar or by tube- 
length, earnest attention must be given to these cor¬ 
rections. It is well to use, in examining the object in 
the microscope, such an ocular as will give about the 
same magnification as will be given by the ocular and 
length of camera on the negative ; beyond this all that 
can be said is that experience, skill, and a knowledge of 
the object are required to get the best correction. 
As examples of medium power work we may mention 
such objects as the fine hairs on the tongue of the 
blow-fly, the dots on a diatom, such as Navicula lyra, 
