E. M. NELSON ON BINOCULARS. 53 



Moreover, these conditions of Abbe's are non-essential, because 

 the most perfect stereoscopic eifect, either "ortho" or "pseiido," 

 is obtained when tlie eye spots are circular. 



The table shows that Carpenter's law in every case holds 

 good, e.g., for orthostereoscopism either " the ordinary view " 

 mast be preserved or transposition must be corrected by re- 

 transposition or a "cross-over." 



In addition to what may be called the ordinary conditions of 

 stereoscopism, Abbe introduces a new and extraordinary one,* 

 viz., that "orthoscopic vision is always obtained when the right 

 half of the right pupil and the left half of the left pu^Dil only 

 are employed." 



With regard to this an editorial note in the " R. M. S. 

 Journal " says if — " Prof. Abbe properly points out what has 

 hitherto not been appreciated, that stereoscopic or pseudoscopic 

 effect does not depend essentially on crossed or not crossed 

 axes, but upon either the outer or inner halves of the pupils of 

 the observer's eyes being put into action in binocular vision." 



I wish to point out that it makes not the slightest difference 

 in the image, whether the whole or a part, whether the top or 

 the bottom, w^hether the right hand or the left hand 23ortions of 

 the pupils of the eyes are utilized. 



Dr. Mercer's method of obtaining orthoscopic vision without 

 eye caps, by making the* iris cut off the inner halves of the eye 

 spots by racking in the tubes, proves the truth of this assertion, 

 because the inner halves of the pupils only are utilized (Fig. 

 10). 



Dr. Mercer's method is entirely opposed to this theory of 

 Prof. Abbe. I have frequently tried it, and find the stereo- 

 scopism quite as strong as with eye caps. 



The duty of the eye caps in Prof. Abbe's orthostereoscopic 

 binocular is to effect a cross-over. Whatever influence they 

 may have on the pupil of the eye is quite immaterial. The 

 contradictory statements in this paper of Prof. Abbe's are 

 remarkable, for, speaking of the arrangement of the semi-eye 

 spots, he says % that "it is quite indifferent whether the effect 

 is obtained with crossing or non-crossing rays, whether the 

 image be erect or inverted or semi-inverted, and whatever 



* " R. M. S. Journal," 1881, p. 204. f " R. M. S. Joiarnal," 1881, p. 299. 

 X " R. M. S. Journal," 1881, p. 204. 



