EXAMPLES OF PROCEDURE. 
91 
Having arranged these matters, we are in a position 
to take steps towards making an exposure. We may 
take, for an example, an object such as the blow-fly’s 
tongue, or a diatom such as a large Aulacodiscus, or, 
perhaps, a section of a “ spine ” of an echinus, and our 
first essay shall be with some low-power objective, as a 
one inch or two-thirds. The magnification may be 
about 30 to 35 diameters, and no ocular is to be used. 
The image is focussed in the microscope in the usual 
way, notice being taken that the lens is not flooded by 
the condenser giving too high an angle ; if we look 
down the tube, having removed the ocular, we shall see 
whether the entire back combination of the lens is filled 
with light ; very few objectives—in fact, none except 
apochromatics—will stand this; so we shut the con¬ 
denser till the back of the objective is about half 
illuminated. We must also make sure that the illumi¬ 
nation is even, and the best way to ascertain this is to 
project the image on to a sheet of white cardboard, 
when any unevenness will be readily seen and easily 
corrected by moving the condenser or bull’s-eye. But 
no such movement will be needed if the previous steps 
have been properly attended to. The microscope is 
now turned to the axial position for photography, the 
junction between the tube and the camera is made, and 
the focussing rod is joined up, if that is the system 
adopted.—The camera being stretched to the extent 
requisite for the desired magnification (see page 93), the 
ground-glass is put in its place and the image, roughly 
focussed, is examined, to make sure that it occupies the 
