CHAPTER IV 

 THE MONOBJECTIVE BINOCULAR 



Construction. — This important instrument may perhaps 

 be regarded as a modification of the Abbe binocular eye- 

 piece (10). It differs in that the main diagonal reflection 

 which divides the light from the objective into two equal 

 parts, occurs at a thinly silvered or platinized surface, 

 instead of at a thin film of air (Swan cube). This metalized 

 surface is made to reflect half and transmit half of the 

 incident light, so that one-half goes to each eyepiece. 

 In Jentzsch's original form, and in Siedentopf's binocular 

 attachment, the hght underwent two reflections on each 

 side. The paths of the light in glass were, of course, 

 equalized on the two sides. 



Three essential matters regarding the monobjective 

 binocular are: (1) the question of parallel or diverging tubes; 

 (2) how best to arrange for the different interpupillary 

 distances of different observers ; and (3) how to allow for the 

 necessary small adjustments of tube length required in 

 scientific microscopy. 



In the original form of monobjective binocular (Jentzsch, 

 1913), two reflecting prisms, separated from the central 

 prism, are respectively attached to the two eyepiece 

 tubes (Fig. 13). Adjustment for the interpupillary 

 distance of the observer is made by moving the two 

 eyepiece tubes and their attached prisms towards and away 

 from the central compound prism. But this also increases 

 or decreases the optical tube length which may thus be 5 

 millimeters wrong. In the two forms of this binocular 

 originated by the optical firm of Zeiss, however (Figs. 14, 

 15), the adjustment for interpupillary distance is made by 

 partially rotating the free prisms, without altering the 



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