E. M. NELSON ON BINOCULAR MICROSCOPES. 371 



cross-over the image would be pseudo- stereoscopic. There is 

 no cross-over in a Stephenson, but then it is an erecting 

 microscope. 



The binocular of the Powell type, which passes the whole 

 pencil, does not possess the first attribute of stereoscopism : the 

 image in both eyes being identically the same. No doubt, 

 owing to the employment of both eyes and for physiological 

 reasons, there may be more or less of a stereoscopic effect, but 

 that is an entirely different thing from true stereoscopism. 

 When, for example, the full moon is observed through a field- 

 glass it appears as spherical as a cricket-ball, the images in 

 each eye must be identical and no true stereoscopism can be 

 present. 



If half the Kamsden's disc above the eye-lens is stopped out 

 by a diaphragm, so long as the cross-over is preserved, the 

 image in an inverting microscope will be ortho-stereoscopic. This 

 was mentioned by "Wenham in 1854; and later, in 1882, Dr. 

 Mercer pointed out that a diaphragm is not needed, but an 

 ortho-stereoscopic effect may be obtained by making the inter- 

 ocular distance less than the interpupillary, which causes the 

 iris of the pupil of the eye to cut off the inner half of the 

 Ramsden disc. 



The disadvantage of a diaphragm above the eye-piece is that 

 it occupies the same place as that in which the eye ought to be; 

 and the disadvantage of Dr. Mercer's method is that the head 

 and eyes must be kept absolutely steady, otherwise there will be 

 a flickering of the image, which causes strain and distress to 

 the eyes : the higher the power, the smaller the Kamsden disc 

 and the greater will be the flickering and strain and fatigue 

 to the eyes. For these causes ortho-stereoscopism in a binocular 

 of the Powell type is of a different character from that of the 

 Wenham or Stephenson type. In books dealing with this 

 subject the Wenham super-eye-piece diaphragm and the Mercer 

 narrow inter-ocular distance are treated as alternative plans, 

 equal in efficiency to the Wenham divided objective method. 

 Such, however, is not the case. It is only necessary to place 

 two microscopes alongside each other, charged with similar 

 objectives and powers, one having a Wenham divided objective 

 and the other a Mercer narrowed inter-ocular distance, when 

 an examination of the same object will at once dispel any theory 



