EXPERIMENTS ON DOUBLE VISION. 201 



XL 



Experiments on Double Vision by Dr. Haldat, Secretary 

 to the Academy of Nancy.* 



TITT7 l« ! «i /• • i • • • • -.a. Vision with a 



F1L. superiority of single vision, or vision with one eye, sm gi e eyevu i_ 



over double vision, or with both eyes, has long been the g-* rl y supposed 

 , . , c , ., . . x, . ■ j • j i- iL to be most dis^ 



■subject ot two opposite opinions. It is maintained by the t j nct 



vulgar, that single vision is most distinct. Philosophers on 



the contrary assert, that wc see better with two eyes, than 



with one. The latter opinion, established by father Che- 



rubin in his treatise on distinct vision, and placed beyond The c ° ntrar y 



1 r J proved by 



all doubt by Dr. J urin, by means of an experiment, which j ur j n . 

 consists in looking at a sheet of white paper, with a piece 

 of pasteboard, or other opake substance, affixed to the 

 tight temple, and projecting so far forward, as to conceal 

 half the sheet of paper from the right eye, while the whole 

 is visible to the left. On looking at the paper alternately 

 With one eye and with both, we perceive very distinctly, 

 that the part seen by both eyes is much brighter than 

 that seen by the left only : the former appears with all its 

 natural whiteness, while the latter appears as if shaded by a 

 thin gauze. Dr. Jurin even estimated the intensity of this 

 obscuration by a very ingenious photometric contrivance. 



If this experimental proof of the superiority of double ^^1^1 

 vision over single wanted farther support, we might adduce 

 the experiments made by means of binocular telescopes, 

 fiic superiority of which hate been acknowledged by all ob- 

 servers, both for distinctness of vision and magnifying 

 power, over single instruments magnifying equally, and of 

 equal clearness. 



From this well established fact, that double vision pro- A double sen- 



..,,,.. . . sation produces 



duces a more vivid and distinct sensation than single, it iol- a single peicep- 

 lows, that the sensation produced by the impulse of light tlon - 

 on one of the eyes is reinforced, if I may use the expres- 

 sion, by that produced on the other ; and that consequently 

 a cpmplex sensation may give rise to a simple perception. 



But does this faculty of forming simple perceptions, when But may two 

 ^ ■. . , ,, . . , . ,, , different sensa- 



the impressions are complex, equally take place in all cases ? t i ons i, e blend- 

 Is it the case when the impressions are heterogeneous, as ed into one? 



* Journal de Physique, Vol. LXIII. p. 387, Nov. 1806. 

 Vol. XVIL— July, 1807. P well 



