i882. 115 Trans. N.Y. Ac. Scu 



substitute one more in accordance with the theory of rapid successive 

 convergency of the optic axes in the estimation of distance. The 

 effect was a decided surprise, in its illustration of the phenomenon, but 

 even more so as a corroboration of the theory. It consisted simply of 

 two concentric circles, respectively i ^ inch and y% inch in diameter 

 as the left-eye picture, and a circle \Y% inch in diameter, enclosing two 

 circles J^ inch in diameter each, with their centres respectively i-io 

 inch to the right and left of the centre of the large circle, as the right- 

 eye picture, the centres of the large circles being about 2^^ inches 

 apart. The expectation was, that the successive combination, so to 

 speak, of the small circles of the right-hand figure, could at least be 

 made so rapidly that a comparison of the sizes could be made at the 

 varying apparent distances. The apparent simultatieousness of the 

 appearance of the near and remote circles, with the expected differ- 

 ence in apparent size, constituted the surprise, which has been ex- 

 perienced by every one upon examining such a slide for the first time. 

 Attention was called to the diagram in a published discussion of some 

 of Sir David Brewster's views of the theory of the stereoscope in 

 1864, in the BritisJi Journal of Photography ; and subsequently a 

 dealer in stereographs placed it among his slides ; but it is more than 

 possible that even those interested in this subject may not have met 

 with it, and I have therefore presumed to call attention to it in connec- 

 tion with the very interesting articles upon the subject before alluded to. 

 Carlisle, Pa., January 30, 1882. 



Mr. Stevens exhibited the stereograph described by Professof 

 Himes, and by large diagrams on paper explained the difference 

 between this and Brewster's method of illustrating the phenomena of 

 the horizontal moon. In addition, he remarked substantially as 

 follows : 



" It gives me much pleasure to present this paper by Professor HiMES, 

 and to call attention to his careful observations on the nature of binocu- 

 lar vision, which were made twenty years ago, and have not received 

 the full notice to which they were properly entitled. The great name 

 of Sir David Brewster carried with it a degree of authority which 

 caused the general acceptance of the geometric theory of binocular" 

 vision. Professor W. B. Rogers,' in this country, had also written 

 a series of very able papers, in which he calculated what must be the 

 form of the externally projected binocular image, assuming each point 

 of this to be determined by intersection of visual lines. Professor 

 HiMES discovered that such intersection is not necessary ; but, unfor- 

 tunately, his paper on this subject was not published in the Atnerican 



I. Am. Jour. Science, vols. XX. and XXI., 1855 and 1856. 



