go 
PRACTICAL PHOTO-MICROGRAPHY. 
after which, if necessary, we remove the disc, or open 
the iris. If the previous operations have not been 
accurately carried out, we may find the disc aperture 
not in the centre of the field ; if there is no great in¬ 
accuracy in this respect we may rectify it by means of 
centring screws for the paralleliser, or, better, by moving 
light and paralleliser together. 
After these dispositions of the paralleliser, the field 
will be brilliantly and evenly lighted all over ; and the 
writer uses this arrangement almost always, no matter 
what power he is using either of objective or condenser. 
Once the light and the bull’s-eye are arranged, they 
need not be moved, and the convenience and certainty 
of having an even lighting are of great consequence. 
To get the best effect from the paralleliser, it should 
not be too near the condenser, eight or ten inches will 
be suitable ; and as the rays are nearly parallel, no 
notable loss of light-power will be entailed. It is well 
to have a cell with parallel glass sides, filled with boiled 
distilled water, between paralleliser and condenser; 
there is no advantage in adding alum to this water. 
4.—The Glass, or Cell of Liquid, 
used as a “screen” or light-filter , maybe placed any¬ 
where between paralleliser and condenser; in an 
ordinary way, the glass used need not, for this position, 
be optically “worked,” but if it is to be close to the 
condenser, or screwed into it, then it should be 
“ worked.” And either cell or glass screen, whichever 
is used, must be at right angles to the optical axis of 
the whole system. 
