1820.] a neiv Theory of Vision. 265 



through a sphere, or convex glass. Take a cylindrical tumbler, 

 fill it with clear water, and hold it in the left handopposite a window. 

 Hold a black slate pencil, or any other slender body, about three 

 inches in length behind this glass vessel; when close, one magni- 

 fied image is seen, but on gently withdrawing the pencil to a 

 greater distance, this image becomes more magnified, and at a 

 certain distance, two images, fully as well defined, are seen at 

 each side of the tumbler ; on continuing to withdraw the pencil 

 two everted images are seen to glide with a considerable 

 degree of curvature towards the posterior surface of the tumbler 

 and at last coalesce into one image, which obliterates the 

 anterior one, or that formed at the anterior surface. This corro- 

 borates the inferences drawn from the former experiment. When 

 the object is near the posterior surface of the, tumbler, the eye 

 receives the rays considerably diverged or magnified ; when the 

 object is at some distance from the posterior surface, the eye 

 receives the rays from the coalesced image formed from the two 

 lateral images. From this experiment there can be no doubt 

 v.'hatsoever that the eye receives rays from two distinct and 

 separate images ; and also that the miiid receives impressions 

 firom a glass globe or convex lens in nearly a similar manner. 

 Should a doubt yet remain, the following experiment may be 

 made : Place a red wafer under one of the planes of a triangular 

 glass prism, resting on a sheet of white paper; we immediately 

 «ee two everted images of the wafer formed in each lateral plane, 



as thus represented /-^^ . The wafer a sends rays or images 



to b and c. As the prism has plane sides, the two images can 

 never come to a focus at any distance ; but if we round off the 

 angle, they immediately unite, and form an oblong image of the 



wafer, as thus represented /^7^^ . 



From these experiments, and many others hereafter to be 

 related, in the second volume of the Experimental Outhnes, not 

 a doubt remained on my mind that reflected erect, and not 

 inverted images, gave mental impressions of a visible world. 

 Surely if any thing can increase our admiration of the power and 

 "wisdom of a Supreme Being, it is the conviction that a beautiful 

 and ever varying landscape is painted in miniature on the trans- 

 parent cornea. When we consider that the black choroid shines 

 through the retina, we should admit that it is very unfit to be 

 the reflecting mirror of the mind. To bring this to the test of 

 experiment, 1 turned out the aqueous, vitreous, and crystalhne 

 humour of the eye of an ox ; on bringing the inverted image of the 

 black letter T pasted on the window to float on the retina, by 

 means of a convex lens, I found that it was perfectly invisible; in 

 some places confused ; indistinct in all. Indeed the retina, were 

 it free frona this and many other objections, and also free from 



