THE EYEriEVE 151 



phragm), as in a high Huyghenian eyepiece. On the other 

 hand if the pupil of the eye is too far below the eyepiece 

 circle (which can only happen with a large eyelens to the 

 ocular), the iris of the eye bounds the field. 



Eyepiece Constant. — The constant or basic number of an 

 eyepiece is the total magnification of the microscope 

 multiplied by the actual diameter in millimeters of the 

 field of view on the object. Since this expresses the apparent 

 size at 250 millimeters of the diaphragm of the ocular 

 magnified by the eyelens, it is the same with any one 

 eyepiece for all objectives. When once determined, it 

 can be readily used to measure the total magnification, 

 by observing an object micrometer. This is one of the 

 easiest ways of measuring magnification. 



Diaphragm. — The diaphragm of an eyepiece determines 

 the size of the field of view^ to which the hghted area of 

 the shde should usually correspond as nearly as possible. 

 In the case of a binocular, the diaphragms of the two 

 eyepieces should correspond in apparent size. This can 

 easily be attained by shghtly shifting one of them, if 

 necessary, in the eyepiece tube. Myopic observers will 

 not focus the edge of the ocular diaphragm unless they 

 wear distance spectacles, (i^stigmatic eyes also must, 

 of course, as already noted, retain their correcting glasses 

 when using a microscope or magnifier.) The corrections 

 of the eyepiece, hke those of the objective, are improved 

 by the use of yellow-green light. 



Measuring Eyepiece. — The measuring eyepiece, with a 

 scale on glass supported by the diaphragm, is usually 

 necessary when large numbers of measurements have to be 

 made. It is important that the same side of the glass 

 circle containing the scale should be turned upwards 

 during a series of measurements, because of the different 

 thicknesses of the glass disc and its balsamed cover. It is 

 easier to make a large series of measurements if the disc is 

 movable by a lateral screw. A probably more accurate 

 method of measuring is to draw both the object and an 

 object scale (stage micrometer) with the Abbe drawing 



