34 
PRACTICAL PHOTO-MICROGRAPHY. 
front of the camera is fitted with a short length of 
metal tube which fits loosely into a cap fitted to the 
ocular end of the tube ; this is to prevent light from 
outside from reaching the sensitive plate during ex¬ 
posure. The inside of the camera must be dead black 
all over ; and close to the plate may be a sheet of 
metal or card having a circular aperture of the size 
desired for the negative, 2f or 3 inches diameter. 
When the camera is stretched to, say, 30 inches, it is 
evident that we cannot examine the image on the 
ground-glass and manipulate the adjustment of the 
microscope at the same time. We require, therefore, 
some arrangement enabling us to reach and operate 
the fine adjustment from the rear part of the apparatus. 
A “ Hooke’s joint ” is used by some, and has been used 
by the writer, but it is far from a satisfactory solution 
of the problem, for when, after focussing, it is laid 
down, there is great danger of the focus being shifted. 
Much better is a rigid rod extending to the back of the 
camera and actuating the fine-adjustment-milled-head 
by means of a cord passed round rod and milled head. 
In the instruments of some workers the cord passes 
round a pulley block on the rod, then round the milled 
head, then round another pulley on the side opposite 
to the rod, then back to and round the rod. This 
arrangement is seen on our own instrument, fig. 8. 
The alternative method—and it is perhaps the better 
—is to counterpoise the power on the rod by means of 
a weight attached to the cord after it has passed over 
the pulley opposite to the rod. Unless, however, the 
