88 
PRACTICAL PHOTO-MICROGRAPHY. 
centred and the condenser must be focussed ; from this 
point variations in procedure may come in according to 
the class of work to be done and the appliances to be 
used. 
3.— The Use of the Paralleliser. 
It will be easily understood that for general work the 
“ streak-image ” is an inconvenient and unsightly one, 
and, as a matter of fact, it is found very difficult to get 
with this image an evenly-lighted held, even when the 
objective, ocular and stretch of camera combined, give 
sufficient magnification to spread the light fairly well 
over the held on the sensitive plate. But we have a 
very simple plan for getting the held evenly lighted, and 
this plan does not damage the quality of the result, even 
for the most critical work. The plan is to interpose 
between the radiant and the condenser a paralleliser or 
“ bull’s-eye.” The nature of this appliance has been 
described, and its use is simple if the principles are 
understood. The light is to be in the principal focus of 
the paralleliser, and to achieve this we turn the fiat, or, 
with the doublet paralleliser, the concave, side towards 
the light, and we place the lens at such a distance from 
the light that a parallel bundle of rays is projected 
forward from the lens on to the condenser. This 
position is constant, and the “ bull’s-eye once in its 
position need not be changed. If a bull’s-eye be placed 
about three inches from the radiant, and a piece of 
paper held so that the rays pass from the lens to the 
paper, it will be found that, when the paper is with¬ 
drawn from the lens, the disc of light either increases 
