192 Prof. Rood on the application, &c. 
the objective, then the left by a suitable brass cap, and taking two 
successive pictures. When using this method it becomes neces-. 
sary to move the mirror towards the right or left hand with each 
successive exposure, which is not only inconvenient, but often pro- 
duces a slight distortion, that prevents the proper stereoscopic 
union of the two photographs. On this account I have generally 
adopted a different plan; the object is placed on an extra stage, 
which can be inclined from 5° to 10° as seen in profile in the wood 
cut, fig. 2; itis photographed first at one angle then at the other. 
In practice the manipulation is easy, and no particular difficulty is 
experienced from the fact that the extreme right and left hand 
portions of the field are thrown slightly out : 
of focus. High and low powers can be used 
equally well. The second negative should 
be taken immediately after the first, before 
the illumination has altered. Ido not know that stereographs 
3. 
F 
by the other hand turning the flap. If the 
collodion is sensitive, a second, or less, suf- 
fices to give an image; if a longer exposure 
be desired the image of the animalcule on the ground glass at G 
can be watched, and the exposure prolonged till the creature be- 
gins to change its position. ®The real difficulty, in the case of liv- 
ing organisms, is found in the fact that all parts of them do not lie 
in the same focus; this in fact is one of the most important aii 
culties connected with the whole subject of microscopic photo- 
graphy. But the introduction of a slight modification in the or- 
inary compressorium, removes it in many cases; the plate of 
glass on which the objects rest, instead of being plane, 18 made 
slightly convex, by the use of a spectacle lens of rather long focus; 
objects to be examined are placed near the point of contact, 
and Lapel applied as usual, when they are brought nearly 
into the same ile. 
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