ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
675 
Slides 1 and 2 are adjustable by hand. Slide 3, on the other band, is 
moved by a screw which can be worked from the interior of the dark 
room. 
The front wall of the dark room, 10*5 cm. from the optical bank, 
carries the projection aperture with its shutter, besides a loop-hole and a 
small velvet curtain for protection against reflections. The projection 
aperture measures 8 cm. in diameter; the shutter is a plate of sheet iron 
movable about an axis. This plate is covered on its outer surface with 
white paper, and thus serves for the proper adjustment of the light, for 
it is easy to recognize on the plate whether the light is evenly distri- 
buted or whether coloured edges, &c., are present. The space between 
objective and projection aperture is covered by a velvet cloth. 
The photographic frame (fig. 75) is moved by a winch along two racks 
on a support, 2’lm. long and 0 • 95 broad, fastened to a fixed stand in the 
dark room. For the author’s purpose of obtaining large photographs 
directly on pnper the following arrangement was employed. Two thick 
glass plates, 80 cm. square, are fitted into two wooden frames in such a 
way that they can be brought almost into immediate contact, so that a 
sheet of paper can be held firmly between them. The two frames are 
fastened together by two screws with bayonet catch. The vertical 
position in the main frame is marked by a stop. The hinder glass plate 
is covered in front with white paper on which the adjustment is made. 
Fig. 75. 
For the purpose of reducing the intensity of the electric light the 
author uses coloured glasses, and almost exclusively dark yellow ones. 
The times of exposure in the author’s experiments varied from 10 
seconds as a minimum to three minutes as a maximum, but were mostly 
from 30-60 seconds. For magnifications up to 200 or 300 the objective 
alone was sufficient, for higher magnifications the large Microscope-stand 
and projection eye-piece of Zeiss were used. 
As regards the choice of the systems, the author, from the experience 
of several years, considers that for low magnifications the Steinheil 
aplanatics leave nothing to be desired. He uses for magnifications of 4-15 
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