THEORY OF THE MICROSCOPE. 
light was lost by reflection from the numerous refracting surfa- 
ces employed. The lenses of each achromatic combination 
were therefore next cemented together, greatly diminishing the 
loss of light by reflection. The compound lenses, used for the 
formation of doublets and triplets, being each separately made 
as nearly achromatic as possible, could be used singly for low 
powers, and two, three, or even four of nearly the same focus 
could be screwed together in a tube, constituting a compound 
objective of high magnifying power. 
French and German achromatic objectives are still made on 
the principle of allowing each achromatic lens to be used sepa- 
rately, though the several combinations of which an objective 
is composed have difierent magnifying powers when used alone. 
15. Angular Aperture. It is necessary to explain dis- 
tinctly what is meant by angidar a^erture^ referring not simply 
to the diameter of the lens, but to the angular divergence of 
the extreme rays of the pencil of light, which a lens is adapted 
to receive. It depends on having the diameter of the lens 
large in proportion to the distance between the lens and the 
object, so that a lens of short focus may have a very large 
angular aperture^ though its absolute diameter is small. 
The amount of light by which any point of an object appears 
illuminated, depends on the angular dimensions of the trans- 
mitted pencil, or, which is the same thing, the angular aperture 
of the object glass. 
If the number of rays of light from any object be insufficient, 
it cannot be seen even though we employ a microscope for the 
purpose. With high magnifying powers the object appearing 
greatly enlarged, the light is spread out over a large surface, 
and, unless the amount of light is proportioned to the magnifying 
power, the object appears dark and imperfectly illuminated. 
With a large angular aperture many delicate markings 
appear, which are quite invisible with a smaller aperture. A 
greater contrast is seen between different parts of the object with 
a large angular aperture, occasioned probably by the fact that 
the lens takes in a larger angular pencil of light from elevations , 
than can issue from minute depressions; differences in the densi- 
ty of diflferent parts of an object produce a similar eftect. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
