EXPOSURE OF PLATE. 73 
an immersion fluid of a lower refractive index, 
é. g. water. 
The light is attenuated after emerging from the 
objective, by the divergence of the rays. The 
angle of divergence depends upon the magnifying 
power of the objective, and the attenuation of 
light will depend upon this angle and the distance 
of the screen on which the enlarged image is 
projected. This attenuation increases as_ the 
square of the distance, while the magnifying 
power — in diameters — increases in simple ratio 
with the distance; e. g. if the magnifying power 
is one hundred diameters at a distance of two 
feet, it will be two hundred diameters at four feet, 
and the time of exposure will be four times as 
great. 
While no precise rules can be given as to time 
of exposure, owing to the number of elements 
upon which this depends, some data may be given 
which will possibly prove useful as hints to be- 
ginners. 
Working with a heliostat in southern latitudes 
(Havana and New Orleans), the time of exposure 
for wet plates, when the amplification was about 
fifteen hundred diameters, and the lens employed 
Zeiss’s one-eighteenth-inch homogeneous immer- 
sion, was from eight to fifteen seconds. (See pho- 
tographs of blood-corpuscles, Plate VI.) 
With Beck’s one-fifth-inch, and an amplifica- 
tion of four hundred diameters, the exposure was 
almost instantaneous. 
At San Francisco without a heliostat, — re- 
