

EXPERIMENTS ON DOUBLE VISION. 9Q5 



After having thus proved, that coloured rays of a dif- Experiments 



ferent nature, obtained by means of refracting mediums, *»* reflected 



' light, 

 produce the perception of a mixed colour by their separate 



action on the eyes ; I was desirous of satisfying myself, 

 whether these effects would equally take place from light 

 reflected by different bodies, and received immediately by 

 the eyes. But as it was necessary for this purpose, that 

 the organs should be placed in such a situation, as to render 

 the impression received by each eye totally unconnected 

 with that received by the other, I separated the bodies sub- 

 jected to the experiment by a thin opake plane, placed per- 

 pendicularly between the two eyes. The little apparatus Apparatus. 

 I employed consisted of a square piece of wood, on the 

 middle of which was placed a very thin vertical plane, the 

 upper edge of which was applied against the forehead and 

 nose, so as to separate the two eyes. The whole of this 

 apparatus, which was twelve or fifteen inches high, was 

 painted black in distemper. The coloured surfaces, the 

 double impression of which was to be observed, were placed 

 parallel to each other on the base, one on each side of the 

 vertical plane. These coloured surfaces were little pieces 

 of pasteboard ten or twelve lines square, painted in distem- 

 per, and representing the primary colours. It is necessary 

 to have some smaller, and some narrower, and particularly 

 to be provided with at least three shades of each colour. 



The apparatus for double vision being placed opposite a Method of 

 window, and the pasteboards on each side of the base, the "sin^it. 

 forehead is to rest lightly on the upper edge of the vertical 

 plane, and then, viewing both objects with great attention 

 at the same instant, the effect of the double impression will 

 be perceived. The phenomena that accompany or precede Phenomena. 

 the complex sensation resulting from it are worthy notice. 

 l f When with steady attention, for about half a minute, 

 or even longer if necessary, we see the objects evidently 

 approach each other, and the plane that separates them The objects ap. 

 disappearing, they- gradually encroach upon each other, P eart0 a P" 

 till they are entirely confounded together, if the distance other, 

 from which they are observed be in proportion to the mag- 

 nitude of the little pieces of pasteboard; as that of twelve 



or 



