On Single and Double Vision, and an Optical Illusion. 73 



ted 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 colors. 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 colors, but 

 by directing the attention to one or the other eye, one or the 

 other color may be made to predominate. Thus a cameleon pic- 

 ture is formed, changing color 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 consid- 

 eration, yet so intimately is it connected with the principles just 

 laid down, that upon its being named certain important conclu- 

 sions 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 cari- 

 cature of bad perspective, in the figure of a barrel with both ends 

 visible at the same time, was not altogether absurd, for if the barrel 

 he 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 evidently 

 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; 

 and as no other apparatus is required than our eyes and the ob- 

 jects of our inspection, it would seem that they were easily made. 

 But it requires rather an acquired power over the organs of vision 

 to be readily successful. Sir David Brewster applied "binoc- 

 ular" convergence upon two figures, drawn side by side to super- 

 impose one upon the other, and compare their exactness in point 

 °f size and form. I have extended the same operation to figures 

 °f unequal size, though of the same form. My son had just 

 completed a half size copy of a drawing representing an Arab on 

 horseback, the correctness of which had been questioned. It was 

 evident that being placed at distances proportionate to their size, 

 the images of the original and copy on the retina would be equal 

 w hen a consistent illusive image might be obtained by conver- 

 gence. The original was hung on the wall, and the half size 

 C0 Py suspended at about half the distance from the observer, at 

 such an angle that one could be fully seen beside the other. I 



Second Series, Vol. VII, No. J9»— Jan., 1849. 10 



