DIAFRAMS IN THE MICROSCOPE. 
39 
As all of the light which reaches the eye must pass through the eye-circle, 
the illumination for eye-circles less than the pupil of the eye in diameter 
will vary with the area of the eye-circle or as 
/ 
\ 
F 2 N* 
in which / is the dis- 
tance of normal vision, 
a the numerical aper- 
ture and .V the magnifi- 
cation as defined on 
page 4 1 . For constant 
numerical aperture the 
illumination decreases 
inversely with the 
square of the magnifica- 
tion. From the above 
it is evident that the 
larger the numerical 
aperture the brighter 
the image, the higher 
the resolving power, and 
the less danger of 
shadows from interven- 
ing dust particles. 
The larger the pupil 
of the eye the stronger 
the illumination. If the 
eye observe a light sur- 
face at the end of a 
darkened tube, the sur- 
face appears consider- 
ably brighter than when 
examined without the 
tube. The tube shields 
the eye from extraneous 
light and allows the 
pupil to dilate, where- 
fore the importance of 
a shield or shade in front 
of the microscope to pro- 
tect the eye from foreign 
light and thus to in- 
crease the illumination . * 
For observations in 
convergent polarized 
light, the arrangement 
of the lenses is indicat- 
ed in Fig. 32. The in- 
8' 
FIG. 32. 
*A convenient shield is a piece of velvet 50 X 100 cni, suspended from a wire stretched across the center 
of the microscope table and about 75 cm. above the tablet This shade can be pushed aside when not desired 
and serves as a cover for the microscope when it is not in use. 
