58 E. M. NELSON ON BINOCULARS. 



vision with the binocular microscope and vision with the anaided 

 eye ; " " notwithstanding this difference the activities of the 

 brain and mind blend the images so as to give rise to sensations 

 of solidity, depth, and perspective ; " "the brain arranges them 

 so that the characteristics of solid vision are still presented," 

 what wonder that the microscopical binocular images are held 

 to be so entirely dissimilar to any ordinary images that it is by 

 brain power alone that they can be turned to any account. 



The truth is that there is no more brain effort required in the 

 examination of binocular objects than there is in recognizing 

 the similarity between two objective boxes which have been 

 made true to one another to yi^ inch. A considerable amount 

 of brain effort would, on the other hand, be necessary to dis- 

 cover their dissimilarity. Let us suppose that the two objective 

 boxes accurately turned with the yi--^ inch of difference are 

 before us on this table, and that I was to tell this Society that 

 " notwithstanding this difference the activities of the brain and 

 mind blend the images so as to give rise to sensations of ' simi- 

 larity ; ' " and again, that " the brain arranges them so that the 

 characteristics of (similarity) are still presented," although 

 these statements are rigidly true, I think you would be justified 

 in denouncing them as high-flown rhetorical nonsense. 



The difference between the laterally displaced images of the 

 microscope and the perspectively projected images of ordinary 

 vision is practically nil (see Figs. 8 and 9, dotted lines), and 

 therefore no more brain effort or activity is required in viewing 

 binocular microscope images than in viewing ordinary objects. 

 It is aperture and focal depth that cause the parallactic displace- 

 ment. If a lens has insufficient aperture the stereoscopic effect 

 will be weak, and on the other hand if the aperture is too great 

 there will be hyperstereoscopism. Thus there is practically no 

 stereoscopic effect with a three-inch objective of 10° of aperture. 



Stereoscopism may be said to begin with a two inch of 15° in 

 an ordinary binocalar ; at the same time it should be re- 

 membered that excellent stereoscopic effects can be obtained by 

 the Cherubin d'Orleans method and three-inch objectives, but 

 in this case we have the axes of the lenses inclined to the 

 object, and on that account true perspective pictures, whereas 

 a semi-aperature of 5° gives no scope for sufficient lateral dis- 

 placement. 



