200 On Single a?id Double Vision, and an optical Illusion. 



of 1846 I communicated the leading principles of what I 

 thought then questionable discoveries, either to Prof. Bache 

 or to Prof. Henry, and consulted him as to their originality. 

 He gave his opinion that they were new. Without the least 

 disposition to contest the point of originality, which I have 

 failed to establish by neglecting to publish my results, I wish 

 merely to inform my friends of what I have in fact done, and 

 thus appear as a collateral witness to the trutii and interest of 

 Sir David Brewster's paper. He has brought forward some 

 points which had never presented themselves to me. That 

 figures less distant than the two eyes may be so viewed as to 

 form an illusive image at a greater distance than the object 

 itself, is evidently true, yet I had never made or anticipated 

 the experiment. Two such small figures might occupy such 

 a situation as to form the pictures on the retina due to a single 

 larger object placed at a greater distance, and thus become an 

 optical equivalent to that object. I am now experimenting on 

 the subject of single vision produced by two identical figures 

 of different colours. So far the results have not excited any 

 very surprising interest. The illusive image, as would be 

 anticipated, usually exhibits the effect of a commingling of the 

 colours ; but by directing the attention to one or the other 

 eye, one or the other colour may be made to predominate. 

 Thus a cameleon picture is formed, changing colour at the 

 will of the inspector. 



Sir David Brewster alludes in his paper to some discoveries 

 made by Prof. Wheatstone, in reference to "binocular" vision 

 of objects of three dimensions. I have not seen the paper on 

 that subject, nor had I turned my attention in the least to its 

 consideration ; yet so intimately is it connected with the prin- 

 ciples just laid down, that upon its being named certain im- 

 portant conclusions at once present themselves. Thus when 

 the hand is held edgewise, within three inches of the nose, 

 one eye will receive an image of the palm and the other 

 of the opposite side ; and the two pictures, being dissimilar, 

 cannot fall on corresponding parts of the retina and produce 

 a single perfect image. Let any one make the experiment, 

 and he will perceive that Hogarth's caricature of bad perspec- 

 tive, in the figure of a barrel with both ends visible at the same 

 time, was not altogether absurd ; for if the barrel be shorter 

 than the distance between the eyes, it is practicable. The 

 same thing will occur with regard to any solid, as a cube, 

 which has several aspects, and the imperfection will be evi- 

 dently greater as the object is smaller and nearer the eye. 



The experiments on this interesting subject can be extended 

 and varied in many ways highly interesting and instructive ; 



