THE MONOBJECTIVE BINOCULAR 53 



tages that a binocular licld-glass has over a moiiocuhir 

 one. In practice it enables microscopical observation 

 to be carried on continuously for hours in comfort. Since 

 the optical tube length cannot be altered much, the high dry 

 objective, if used, should have a correction collar. A high- 

 power water- immersion objective with a correction collar 

 may be of advantage. A binocular attachment is made by 

 most opticians, and by its aid most monocular microscopes 

 can be readily changed into binoculars. In the writer's 

 experience, this change is worth making. The monobjec- 

 tive binocular might well be improved for accurate micro- 

 scopy by graduating the sides of the eyepieces, or by 

 fitting them in graduated sliding tubes about 3 centimeters 

 long; so that corrections for cover-glass thickness with 

 oil-immersion objectives could readily be made. Of the 

 three forms known to the writer: Jentzsch's pattern, with 

 two reflections and slightly variable tube length; Sieden- 

 topf 's model in the Bitumi attachment, with two reflections 

 and fixed tube length; and the new model of Zeiss, with 

 doubtless four reflections and fixed tube length; the Bitumi 

 model would apparently be optically the best. 



Practical Points 



1. For stereoscopic vision with magnifications below 100, 

 either the Greenough binocular or the monobjective binocular 

 with halved eyepiece circles can be employed, or the erecting 

 Lihotzky attachment may be used. 



2. The microscopist should train himself to the use of parallel 

 tubes, for they have several advantages over converging tubes. 



3. For scientific microscopy, change of distance between 

 eyepoints in Jentzsch's model of the monobjective binocular, as 

 it slightly alters the optical tube length, should be compensated 

 by moving the eyepieces up or down. (If they cannot be moved 

 down, a few millimeters may be cut off each tube and replaced by 

 a collar; for high-power work especially.) 



4. There should also be some way of altering the optical tube 

 length of all monobjective binoculars slightly, to adjust the 

 high-power oil-immersion objectives for optimum results; unless 

 cover-glasses 0.17 millimeter thick are always used. It is 



