rilE MONOBJECTIVE BINOCULAR 



47 



optical tube length. In the Jentzach form, the two 

 eyepiece tubes might work a spiral fitting when adjusting 

 for interpupillary distance. It could perhaps be arranged 

 that an increase or decrease of 10 millimeters from the 

 normal (65 milhmeters) would cause a depression or eleva- 



FiG. 13. Fig. 14. 



Fig. 13. — Diagram of the prisms in the original Jentzsch monobjective 

 binocular. The double line indicates the semireflccting surface of silver or 

 platinum. The interocular distance is altered by shifting out or in the two 

 outer prisms, which are attached to the eyepieces. This of course increases or 

 decreases the optical tube length. Note that there are two reflections on each 

 side. 



Fig. 14.- — Diagram of arrangement of prisms to be used without altering the 

 optical tube length. Adjustment for interocular distance can be made by 

 rotating the left-hand prism. This is doubtless the arrangement used in Sieden- 

 topf's binocular attachment, Bitumi. Note that there are two reflections on 

 each side. This arrangement is preferred by the writer. 



tion of the eyepieces of 5 millimeters, and the optical tube- 

 length would be unaltered. 



Observers accustomed to the ordinary Greenough binoc- 

 ular may sometimes prefer converging tubes in the 

 monobjective binocular also. Converging tubes may be 

 presumed to suit best with eyes accommodated for reading 



