60 E. M. NELSON ON BINOCULARS. 



resolutiou in a vertical direction equal to that 

 in a horizontal direction. 



The method and kind of illumination make 

 an enormous difference with the stereoscopic 

 binocular. Speaking generally reflected light 

 by lieberkuhn or side reflector and dark ground 

 illumination with an achromatic condenser and a stop yield 

 excellent results. In this latter case I prefer not to use a 

 bull's-eye. 



Interesting experiments may be performed by passing a strip 

 of paper with a hole in it across the back of the objective 

 through the slotted nose-piece (Fig. 6) ; a somewhat deep ob- 

 ject should be on the stage, and a monocular body used. By 

 drawing the hole from the centre of the objective to its 

 periphery, the lense being carefully focussed on a middle plane 

 of the object, the upper part of the object within the focus is 

 displaced one way and the lower part beyond the focus is dis- 

 placed the other way. Bastard stereoscopisra may be seen in a 

 monocular by making two holes in the paper strip, so that two 

 marginal pencils alone are passed (Fig. 5). If, with a stereo- 

 scopic binocular and a lens of 80°, a similar stop be used hyper- 

 stereoscopism will be the result, but if the two holes be brought 

 to the centre there will be hardly any stereoscopic effect, the 

 best results being obtained when a stop as in Fig. 4 is used. 



The position subsequently taken up by Carpenter in connec- 

 tion with the binocular microscope is very strange. In his 

 writings on the subject, prior to the publication of Prof. Abbe's 

 papers, we find a lucid and accurate (excepting only the loose 

 use of the word perspective) explanation of the phenomena, 

 but when he criticized the Abbe eye-piece * at the R. M. S. he 

 contradicted all his former writings by saying that it was not. 

 an orthostereoscope, notwithstanding that the conditions were 

 precisely the same as in the case of Wenham's and Nachet's I. 

 and II. (see Table). The only explanation I can suggest is 

 that he did not trace the path of the rays, and in consequence 

 he failed to perceive that a cross-over had been effected by the 

 eye caps. 



Prof. Abbe has rightly pointed out in another paper f that 



* " Journal R. M. S.," 1880, p. 1088. 

 t "Journal R. M. S./' 1884, p. 26. 



