COMPOUMD ACHROMATIC MICROSCOPE. 
13 
from each other, equal to half the sum of 
their focal lengths, and with a stop or 
diaphragm placed midway between the 
lenses. Huyghens intended this eye-piece 
to diminish the spherical aberration, like 
the positive eye-piece, and especially to 
enlarge the field of view, both valuable 
qualities, but he- was not aware of the 
most important excellence of his inven- 
tion. It was reserved for Boscovich to 
show that he had by this important 
arrangement accidentally corrected a 
great part of the chromatic aberration, 
as will be shown hereafter. (See 30.) 
The negative eye-piece is therefore the 
one generally employed for the micro- 
scope. 
26. A Section of a modern Com- 
pound Achromatic Microscope, is 
shown at Fig. 4, where 0 is an object, 
and above it is seen the triple achromatic 
objective. The lenses E E, and F F, con- 
stitute the negative eye-piece invented 
by Huyghens. The piano convex lens 
E E, is called the eye-glass, F F is the 
field-glass, and between them, at B B, 
is a dark stop or diaphragm. 
The course of light is shown by the 
three rays drawn from the centre, and 
three from each end of the object O ; 
these rays, if not prevented by the lens F 
F, or the diaphragm at B B, Avould form 
an image at A A ; but as they meet with 
the lens F F, in their passage, they are 
converged by it, and meet at B B, where 
the diaphragm is placed to intercept all 
Fig. 4. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
