Visioti, tracing them to Functional Actions of the Brai7i. 251 



tions of right lines drawn from their images on the retina 

 through a certain point which has been termed the centre of 

 visible direction. Hence supposing c and (/ to be these points 

 in the two eyes, tlie sh'p of paper S, by the operation of this 

 law, will be perceived by the right eye somewhere between 

 the lines mc, 7ic produced, and by the left eye somewhere be- 

 tween m' c', n' c' produced. If S appears at the distance of the 

 point in which the axes of the two eyes meet, then two images 

 S' and S" will be seen when the eyes are directed to P; the 

 two images will overlap each other when the axes meet in P', 

 and they will coalesce into one object when the eyes are di- 

 rected to S. 



These things being premised, we shall now proceed to state, 

 as briefly as possible, the experiments by which the phaeno- 

 niena that require particular attention may be exhibited. 



Exp. 1. The two eyes being equally sensible to light, and 

 the two lights F and F' being placed as in the figure, make 

 the slip of white paper S appear double, by directing both 

 eyes to a distant point P. Between one of the eyes L and the 

 light next it interpose the screen C D, the right eye alone 

 being exposed to the direct light F. As soon as this unequal 

 exposure of the two eyes is made, the two images of S will be 

 observed to change their colour ; S', which is seen by the ex- 

 posed eye, becoming g-rcd???, and S'^, which is seen by the shaded 

 eye, becoming red. Withdraw the screen C D and intei-pose 

 the screen A B; when L, which is now the exposed eye, will 

 see the image S" change from red to green-, while S', which is 

 now the image seen by the shaded eye, will change from green 

 to red. The same phaenomena occur, and are even more vivid, 

 if one of the eyes is exposed to the rays of the sun either di- 

 rect or reflected. 



ILx-p. 2. In the first experiment the images of S fall on the 

 retina out of the axes of the eyes a and a' . If this circumstance 

 should be suspected to have any part in producing the green 

 and red appearances, direct the two eyes to S, thus bringing 

 the two images m n, m' n' into the axes. When S is seen by 

 both eyes at once, in this case it appears of its proper white 

 colour ; but if the screen E G is turned alternately to the po- 

 sitions G ^, G d! ^ so as to make S visible to onJy one of the 

 eyes at once, then it will be found to appear ^'r^r/i to the ex- 

 posed eye, and red to the shaded one. 



This observation is an imjiortant one in another point of 

 view, as it proves that the red and green colours of the two 

 images are com])lementary to one another, a fact which is de- 

 monstrated otherwise by directing the two eyes to P' ; for the 

 part h c, where the two images overlap each other, appears 

 while, though the rest ol" them n c, h d, appear red and green. 

 2 K2 



