14 ELEMENTARY CHEMICAL MICROSCOPY 
position for the pupil of the eye when looking into the micro¬ 
scope. If either above or below the eye-point, light rays are 
lost and the image is less bright and less clear. The diameter 
of the eye-point is dependent upon the numerical aperture of 
the objective and the magnification of the microscope. It will 
be found upon measuring the diameters of the eye-circles pro¬ 
duced by different oculars with the same objective, that they 
are inversely proportional to the magnification obtained and 
that with different objectives and one and the same eyepiece, 
the diameter of the eye-circle varies directly as the numerical I 
aperture of the objectives. The value of the numerical aperture | 
in any consideration of the probable performance of different I 
objectives of the same equivalent focus has already been alluded 
to. We now see that there is a close relation existing between 
numerical aperture and the performance of the ocular; for 
example, of several objectives of approximately the same equiv¬ 
alent focus, but possessing different numerical apertures, that | 
one having the highest aperture will permit the employment | 
of an ocular of much higher power and thus yield a considerably 
greater magnification without loss of detail. I; 
If an attempt is made to increase the ocular magnification 
beyond a certain limit the eye-point becomes so small that the 
image resulting is blurred and indistinct. This fact must be 
borne in mind in microchemical examinations where high 
magnifications must often be brought about by using high- 
power oculars with low-power objectives of long working dis- 
i'.f 
tance. jj| 
In order that images of satisfactory distinctness and sharp- i 
ness of detail may be obtained, the optical combination for work iL 
must be such as to yield an eye-point not less than one milli- | 
meter in diameter nor greater than the diameter of the pupil of | 
the eye of the observer.^ The diameter of the eye-point and | 
the position of the plane in which it lies can easily be ascertained 
by holding a piece of thin ground glass or waxed paper over the 
ocular, shading it with a screen or with the hand and raising 
1 Wright, F. E., The Methods of Petrographic Microscopic Research, Bui. 158, 
Carnegie Inst. Washington, 1911, p. 38. 
