THE TWIN-OBJECTIVE BINOCULAR 39 



If the interpupillary distance is slightly different from 

 the distance between the eyepoints of the eyepieces, part 

 of the eyepiece circle in each eyepiece, or in one only, may 

 fall outside the pupil of the eye, and there may be a loss of 

 light and aperture. It might thus be useful if a clamp were 

 provided to secure the distance between the eyepoints 

 unaltered for any constant user and any pair of eyepieces. 

 The eyes, however, must be kept at a fixed distance from 

 the eyelenses (usually quite close to them) to retain the 

 correspondence between interpupillary and eyepoint dis- 

 tances. The correct distance between the eyepieces is 

 attained when there just appears a complete circle to 

 each eye when the other eye is closed. In some binoculars, 

 there is a shutter for each eye (126) to aid in this adjustment. 

 The correct distance between the eyepieces gives the max- 

 imum of hght to both eyes together. The distance between 

 the eyepieces can be measured by laying a flexible milli- 

 meter rule across the eyepieces from the outer edge of one 

 to the inner edge of the other. 



It is evident, from the construction of the twin-objective 

 microscope, that the focusing of a plane object will only 

 be correct along the median line from front to back. So, 

 on right and left, the focus of one tube will be more or less 

 above, and the focus of the other tube below, the plane of 

 the object (Fig. 11). This puts a limitation on the use 

 of the instrument with medium or high powers, and ren- 

 ders it fitted for viewing sohd, rather than plane, objects. 

 If used for plane objects, its stereoscopic effect is of small use. 



The adjustment of the instrument can be tested with a 

 plane object, such as a printed card. If the adjustment is 

 correct, both tubes should be simultaneously in focus at the 

 center front-to-back lines of their fields, and the contents 

 of the two fields should coincide to the margins. With 

 low powers, the correct focus for each tube can be ascertained 

 by putting a printed card on the stage and closing one eye. 

 At the correct focus for either tube, the center line of the 

 field is sharp, while both right and left sides are more or 

 less dim. 



