32 PHOTO-MICROGRAPHS. 
its axis in a line with that of the instrument, so 
that none but parallel rays reflected from the sky 
reach the object, or the back lens of the achro- 
matic condenser, if one is employed. 
It may be given as an absolute rule that no light 
should reach the screen except that which comes from 
behind the object ; and it is always desirable to shut 
out, by means of diaphragms at the back of the 
object, all light not directly concerned in the illu- 
mination of the field to be photographed. 
In photographing with comparatively high pow- 
ers without a heliostat, wide-angled immersion 
lenses possess a special advantage, from the fact 
that they admit more light than lenses of less 
angular aperture having the same magnifying 
power. 
It is evident that the light which enters the 
objective will be more or less attenuated accord- 
ing to the distance between the screen and the 
object-glass, and this attenuation is as the square of 
the distance, while the increase in magnifying power 
—jin diameters — varies in a simple ratio, corre- 
sponding with the increase in distance. Thus if at 
a given distance the amplification is one hundred 
diameters, at twice this distance it will be two hun- 
dred, and the light will have but one fourth the 
intensity. This should be borne in mind in esti- 
mating the time of exposure required with the 
same lens and light, with the screen at a greater 
or less distance from the object-glass. 
Under the law above stated, the attenuation of 
light becomes so great for high powers that illu- 
