PHOTOMICROGRAPHS AND LANTERN SLIDES 
147 
the condenser until the lighted circle on the ground glass is very small. 
Then shift the light or the condenser, for it is possible that the con¬ 
denser is also off center, or both until the illuminated circle is 
centered. After the light source, the condenser, and the objective 
have been brought into line, rack the condenser back and forth until 
the beam of light is focused on the object to be photographed. 
Then make certain that the cone of light at the point where it enters 
the objective has a diameter great enough to fill the aperture of 
the objective. The ordinary form of Abbe condenser is not likely 
to be satisfactory with objectives of 16-mm. focus, and should not be 
used at all with objectives of longer focus. 
Focus the object upon the ground glass. Even with a 16-mm. 
objective, the ordinary ground glass is rather coarse for accurate 
focusing. Always examine the image with a focusing lens. A 
brilliant view may be obtained by fastening a thin cover-glass to the 
ground glass with a small drop of balsam. At this spot the image 
may be examined very critically. Of course, as in any photography, 
the ground side of the glass should be nearest the object, occupying 
exactly the place which is to be occupied by the emulsion side of the 
plate. Do not focus indiscriminately, but be sure that the image 
is sharp at the level of the ground side of the glass. It is a good plan 
to make a cross upon the ground glass with a pencil or pen, and then 
add a drop of balsam and a cover-glass. Focus on this mark and 
fix the focusing glass at this level. The cheap tripod lenses are good 
for this purpose. 
The time of exposure will vary with the magnification, the 
intensity of the light, and the speed of the plate. The exposures will 
be much longer than in ordinary photography. It is better to use 
artificial light, since one can more quickly learn to estimate the 
length of exposure when the intensity of the light is constant. A 
slow plate, even the very slow contrast plate, is likely to prove most 
satisfactory for the beginner. With a Welsbach lamp, a contrast 
plate of the same speed as a lantern slide, and a 16-mm. objective 
used without an ocular, or Abbe condenser, try an exposure of 30 
seconds. Develop the negative in whatever solution is recommended 
in the directions which come with every box of plates. If the negative 
is too weak, make a longer exposure; if too dense, shorten the 
exposure. A little experience with your apparatus will soon enable 
you to estimate the length of exposure with some certainty. We 
