THE TWIN-OBJECTIVE BINOCULAR 43 



are interchangeable, but those by different makers are 

 usually calculated somewhat differently. 



Tests. — A printed card is a good test for focal differences 

 and differences in extent of field. To test the resolution, 

 diatoms have been used by the writer, such as the coarser 

 forms of Moeller's test slide of twenty. It is useful to 

 test the instrument to the limit occasionally; since other- 

 wise, in the course of ordinary work, defects in the image 

 may go unnoticed for years. In all testing, the two fields 

 should be examined for coincidence as to extent, and the 

 focuses of the two tubes at the center of each field should 

 be tested for identity (with distance vision). 



Literature. — The Greenough, or twin-objective, binoc- 

 ular was introduced in 1897. The original account of it 

 by Czapski and Gebhardt may be found in the Zeitschrift 

 fuer wissenschaftliche Mikroskopie, for that year. 



Orthomorphy. — This term applies to binocular vision 

 through an instrument which resembles ordinary binocular 

 vision without an instrument. No microscope gives it 

 correctly. Theoretically, perfect orthomorphic vision 

 requires conditions which cannot be fulfilled (Von Rohr) ; 

 and by so much as they fail of fulfilment, the orthomorphic 

 effect is impaired. A nearer approximation to ortho- 

 morphy can be made by putting diaphragms in front of the 

 twin objectives (Zeiss); but these have mostly been dis- 

 continued, since they lessen the aperture. How far the 

 presumable disadvantages of wide departures from ortho- 

 morphy are counterbalanced by the advantages of larger 

 apertures and higher magnifications is to be decided by 

 practice. The trend at present seems to be to increase 

 both apertures and magnifications. As already stated, 

 stereoscopic effect does not seem of great scientific impor- 

 tance, however pleasant and convenient. 



(By Greenough's reflecting prisms under the object 

 [Zeiss], the lateral and under parts of a small object, such 

 as an insect, can be seen with the binocular after the 

 upper surface has been examined.) 



