8 4 
PRACTICAL PHOTO-MICROGRAPHY. 
stage; in this case it is moved back and forward till 
the proper position is found for it. This method of 
using the “ bull’s-eye ” is for very low power work only; 
for instance, it works well with the iin. (aa) of Zeiss 
and similar lenses. 
The paralleliser (or bull’s-eye) of whatever type 
should be fitted with either a good “ Iris ” diaphragm, 
or an opaque disc having a small central aperture ; this 
l 
Fig. 27. 
will be found necessary for centring the paralleliser, and 
for focussing the condenser to the parallel bundle of 
rays; the focus for parallel rays being, of course, shorter 
than for diverging rays. Without some such device as 
that suggested, we should be unable to adjust this focus, 
as the parallel rays form no image—of the radiant, for 
instance—on which we could focus. 
The gas-burner, known as the “ Welsbach,” yields a 
light of great actinic value, and it may be used for low- 
power photo-micrography, but it will be found that 
when the condenser is in focus, the image of the 
“ mantle ” will obtrude itself upon the held ; this is, to 
