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 

 aperture of the objectives. The value of the numerical aperture 

 in any consideration of the probable performance of different 

 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. 



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- 

 tance. 



In order that images of satisfactory distinctness and sharp- 

 ness of detail may be obtained, the optical combination for work 

 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. 1 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. 



