EXPOSURE OF PLATE. 71 
A square diaphragm, covering one half the 
plate, may be used for the purpose of bringing 
two different pictures in juxtaposition on the 
same plate. The first exposure is made with one 
half the plate uncovered ; the second, after chang- 
ing the diaphragm to the previously uncovered 
side. By this means, different objects which it 
is desired to compare are brought together and 
printed from the same negative, —e.g. the blood- 
corpuscles of man and of the dog; or the same 
object may be taken on the two sides of the plate 
with a different amplification. This method is 
useful also for comparing the performance of dif- 
ferent objectives ; for the conditions being identi- 
cal as to time of exposure, development of plate, 
printing of positive, etc., differences of definition 
must be ascribed entirely to the optical part of the 
apparatus. 
IX. EXPOSURE OF PLATE. 
No definite rules can be given as to time of 
exposure, and the beginner must be prepared to 
expend considerable material in learning to esti- 
mate the amount of light by an inspection of the 
screen when all is ready for the exposure. 
The elements upon which the result depends, 
as seen in the illuminated field upon the ground- 
glass plate, are: the source of illumination, — sun- 
light or artificial light; the use of a heliostat, —— 
direct light, or light reflected from the sky; the 
