ZOOLOGY AN1) BOTANY, MICROSCOPY, ETC. 
Ill 
into a side tube through which the living object is observed. At the 
moment selected for the exposure, the prism is rapidly pushed aside, its 
motion being followed by a slit cut into the movable portion of the 
shutter attached to the objective, so that the plate enclosed in the 
camera is illuminated for an instant. 
The disadvantages of the method are the blinding of the eyes by 
the intense light when watching for the most favourable moment for 
exposure, and the continuous exposure of the object to light and injurious 
heat rays. 
The second method consists in the employment of a source of light 
which flashes up instantaneously. For this purpose magnesium powder 
is used ; but the method is improved by adding to the powder sub- 
stances which render the flame greenish-yellow, so that photographs can 
be obtained with but imperfectly corrected lenses. The objection to 
this method is the impossibility of selecting the moment of exposure. 
The third method, designed by the author in order to obviate tlio 
difficulties in the preceding methods, is necessarily more complicated 
and consists of two principal parts. The first part is represented by 
a slide shutter interposed between the source of light and object and 
showing successively : — 
(1) A circular opening covered with coloured glass or gelatin, by 
which the light and heat rays can be reduced as much as is desired. 
(2) A rectangular slit which makes the 'exposure by sliding for- 
ward. 
(3) A non-perforated portion of the shutter which excludes all light 
after the exposure is made. 
The second part of the apparatus consists of another slide carrying 
a reflecting prism by which the light is thrown into a side tube. When 
the slide is released the prism is rapidly pushed aside, and passage is 
opened for the light to the main tube. Both shutters must be pneuma- 
tically released at exactly the same time. The prism-carriage, which is 
Fig. 9. 
a- 
d < Taffeta strip. _ 
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7r. 
best fastened directly behind the objective, then moves rapidly aside, 
opening a path for the light rays to the sensitive plate ; the slit of the 
first slide passes across and the exposure is made. 
The only difficulty which can occur in this method is insufficient 
rapidity of the two shutters. 
The author makes an improvement on the ordinary Anschuetz 
