THE MONOBJECTIVE BINOCULAR 49 



instruments of at least one maker, without altering the 

 optical tube length, by the rotation of two prisms, or 

 prism sets. Finally, the optical tube length can be 

 increased by pulling out the eyepieces sufficiently, without 

 altering the distance between eyepoints (thus correcting 

 for cover-glasses below 0.17 milhmeter thick); though 

 there is in these binoculars no means of shortening the 

 optical tube length, except by removing the nosepiece. 

 (Hence objectives with correction collars are especially 

 useful here. ) 



Stereoscopic Effect. — To obtain, with this binocular, 

 stereoscopic vision, we can use half-discs in the eyepiece 

 circles ; or we can shorten the distance between eyepoints, so 

 that the inner half of each eyepiece circle is cut off by the 

 iris of that eye. In both cases, the image on the retina is 

 deteriorated, as compared with non-stereoscopic vision, 

 since the rays are unilateral. The effect on each eye is the 

 same as if one semicircle of the objective aperture was cut 

 off. The transverse aperture is, of course, halved. Hence, 

 useful magnifications are restricted to 500 times the work- 

 ing aperture. When using the half-discs, it seems good to 

 have somewhat more than a half-circle of light for the best 

 eye; and correspondingly somewhat less than a half -circle 

 for the other eye. However, we may also proceed, as Abbe 

 himself recommended (10, 12), by halving only the left 

 eyepiece circle; when a sufficiently strong stereoscopic 

 effect for most purposes will be attained, w^ithout loss of 

 aperture or other marked deterioration of the image, and 

 without cutting off more than one-quarter of the light 

 (the right eye being regarded as the usual observing eye). 

 (This is also done in the Lihotzky binocular attachment, 

 made by Leitz.) But these binoculars are best used at full 

 aperture, without attempting to secure a strong stereoscopic 

 effect; so that the comfort of observing with both eyes 

 can be attained along with perfect resolution. There will 

 still be a varying stereoscopic effect, arising doubtless 

 from small lateral movements of the head. This slight 

 stereoscopic effect may be sufficient for many purposes, 



