Experiments ow a new Theory of Vision. 53 



difference in the inverted image, which they conceived to float on 

 the retina; neither could the crystalline jump backwards and 

 forwards to accommodate the eye to the object at different di- 

 stances. Indeed I cannot conceive the cause of this jumpmg of 

 the lens. If the distance of the object be ascertained, and con- 

 sequently the object seen before the lens with its thousands and 

 tens of thousands of muscles begins to jump, what occasion is 

 there for that movement? But if the jump take place before the 

 object is seen, how can the extent of the jump be ascertained ? 

 Look before vou leap, is a common but a necessary piece of axi- 

 vice. The fact is, that the eye principally judges of different di- 

 stances, bv comparing the visible size of the corneal image with 

 the educated sense of the tangible object, intervenmg objects, 

 strength of colouring, &c. In viewing a painting, the objects are 

 all equidistant on t1ie canvass; vet we conceive them to beat 

 relative distances. The following easy experiment may also show 

 that the rays diverge in passing through a sphere or convex glass. 

 Take a cvlindricaf tumbler, fill it with clear water, holdujg it in 

 the left hand opposite a window. Then hold a black slate pencil; 

 or any other slender body, about three inches in length, behind 

 this glass vessel. When close, one magnified 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 con- 

 tinuing to withdraw the pencil, these two everted images are seen 

 to glide with a considerable degree of curvature towards the po- 

 sterior surface of the tumbler, and at last coalescing into one 

 image, obliterate the anterior one, or that formed at the anterior 

 surface. This corroborates the inferences drawn from the former 

 experiment. When the object is near the posterior surface of 

 the tumbler, the eye receives the rays considerably magnified or 

 diverged ; but when the object is at" some distance from the po- 

 sterior surface, the eye receives the rays from the united lateral 

 images. From this experiment, there can be no doubt whatsoever 

 that the eye receives rays from two distinct and separate images, 

 and also that the mind' receives impressions from 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 triangu- 

 lar glass prism, resting on a sheet of white paper on the table, 

 we immediately see two everted images of the wafer formed in 



each lateral plane, as thus rqiresented : ^r^f '^'^® wafer a 



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 from 



the eye ; but if we round off the angle, they immediately unite, 



D 3 a»d 



