THE MONOB.JECTIVE BINOCULAR 51 



When the right distance between eyepoints is attained, there 

 should be left a minimum of stereoscopic effect (with 

 parallel tubes). The attainment of this minimum can be 

 used as a test for the right distance between eyepieces. 



The optical length of the two eyepiece tubes should, 

 of course, be the same. If there is a shght difference in this 

 respect between the tubes, or if the focal lengths of the 

 two eyepieces differ, it may be adjusted by puUing out 

 one eyepiece a Uttle. But if the eyes of the observer differ, 

 appropriate distance spectacles should be worn; or correct- 

 ing glasses should be fitted in caps over the eyepieces. 

 The fields of the two eyepieces should be identical in size. 

 A shght difference in this respect may be remedied by 

 sUghtly moving the diaphragm in one eyepiece. In some 

 makes of binocular, one eyepiece can be focused by a 

 spiral fitting. If this is not attended to constantly by 

 each user of the instrument, it is liable to be a source of 

 error, and may waste time. A milHmeter difference in 

 position of the eyepieces with regard to tube length will 

 slightly injure binocular vision with the highest objectives 

 in correct adjustment. 



Binoculars with parallel tubes are usually calculated for 

 use with a resting normal eye, or with distance spectacles. 



Optical Tube Length. — Those monobjective binoculars 

 in which the eyepieces along with the prisms alter their 

 distance from the objective along the course of light for 

 different eyepoint distances, should, it seems, for accurate 

 work, have some arrangement for compensating this 

 by drawing out or pushing in both eyepieces. (Perhaps 

 the eyepieces might be depressed enough automatically 

 by a spiral fitting as the eyepiece tubes are moved farther 

 apart.) For all binoculars, some means of slightly shorten- 

 ing the tube length is occasionally needed for accurate 

 high-power work. Correction for cover-glass thickness 

 (too thin covers), with oil-immersion objectives, can be 

 made by pulling out the eyepieces sufficiently. 



Advantages and Disadvantages. — For magnifications 

 over 50, or so, to about 300, the monobjective binocular 



