ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



271 



under side of the stage, for shutting off all light except a small pencil 



from the mirror. It may be mounted in either of two forms : the one 



to fit into the usual tube, which, 



in the cheaper Microscopes, is ^'^g. 55. 



attached to the under side of the 



stage, the other to screw directly 



into the stage aperture. 



The device is shown in Fig. 55. 

 The milled edge serves to rotate the 

 plate with the spiral slot over the 

 radial slot (shown by dotted lines), 

 thus giving varying degrees of 

 obliquity. 



Stereoscopic Vision with Non-stereoscopic Binocular Arrange- 

 ments. — It will be remembered that in his paper " On the Conditions 

 of Orthoscopic and Pseudoscopic Effects in the Binocular Micro- 

 scope," * Professor Abbe pointed out that an orthoscopic (stereo- 

 scopic) effect was produced if the inner halves of the " Eamsden 

 circles " just above the eye-pieces were shut off by diaphragms (that 

 is like 0, Fig. 56), and a pseudoscopic effect when the outer halves 

 were so dealt with (that is like P, Fig. 57). 



Fig. 56. 



Fig. 57. 



D 



D 



a 



Fig. 58. 



Dr. A. C. Mercer, of Syracuse, U.S.A., points out that this explan- 

 ation solves a difficulty which has perplexed many microscopists, and 

 has hitherto remained unexplained. Powell and Lealand's high- 

 power binocular is essentially non-stereoscopic, and theoretically ought 

 not to give any appearance of relief to the objects. It has nevertheless 

 been frequently observed that a distinctly stereo- 

 scopic effect was obtained, and this was attributed 

 entirely to the imagination of the observer. Dr. 

 Mercer, however, shows that it is a true and not an 

 illusory effect, and that it depends upon the extent 

 to which the eye-pieces are separated. 



When the eye-pieces are at such a distance apart 

 that the Eamsden circles correspond exactly with 

 the pupils of the eyes, centre to centre (Fig. 58), the 

 object appears flat. If, however, they are racked 

 down so as to be somewhat nearer together, the 

 centres of the pupils fall upon the outer halves of 

 the Eamsden circles, and we have the conditions 

 for orthoscopic effect ; while if they are racked 

 up so as to be more separated the centres of the pupils fall on the 

 inner halves and we have pseudoscopic effect. 



This is quite in accordance with what takes place in the use of 



* See this Journal, i. (1881) pp. 203-11 (.3 figs.). 



