Experiments on a vew Theory of Vision. 55 



riuni. Indeed, the idea that the mind or sense could travel beyond 

 the cornea, ride on the whirlwind, and, like a fairy Mab, measure 

 invisible angles of an invisible and distant image, is so very in- 

 consistent, that we cannot but express surprise at its adoption. 



If a man were graveU" to say that he could touch the moon, he 

 would be looked on as mad : but an astronomer says, that on 

 looking through a telescope he can measure the invisible image 

 of that body nearer to the eye than the moon, and beyond the 

 influence of the nerves, and the astronomer gets credit for the 

 assertion. As the knowledge of distance almost entirely arises 

 from experience founded on the analogy between the sense of sight 

 and touch, tlie former at a verv early period of existence is in- 

 adequate to regulate our perceptions. When an infant begins to 

 notice, natural education commences, external, objects are the 

 letters, the nerves the instructors of themvnd: the insufficiency 

 of sight is evident by the anxious desire to feel and to examine 

 every nev/ plav-thing ; the image of tlie rattle is delineated on 

 the cornea, and the child, believing it to touch the eye, grasps at 

 it although far removed. On the same principle, I have heard 

 a child cry bitterly for the mnnn to play vvith. In a few months 

 the sense of touch has partly educated the eye in judging distance 

 by the apparent magnitude of the corneal image. A man born 

 biind and suddenly restored to sight would suppose every object 

 to touch his eye. 



All that is accomplished by telescopes and microscopes (ac- 

 cording to the retinal theory) is first to make an image of a di- 

 stant object bv means of a lens, and then to give the eye some 

 assistance for viewing that image as near as possible, so that the 

 angle which it shall subtend at the eye may be very large com- 

 pared with the angle which the object itself would subtend in the 

 same situation: — this is done by means of an eye-glass, which 

 so refracts the pencils of rays, as that they may afterwards be 

 brought to tlieir several foci l)y the natural humours of the eye. 

 Now it is evident from the foregoing experiments tiiat this tlieory 

 is fallacious, and that a telescope, as shall hereafter be more fidly 

 shown, docs nothing more than diverge the rays, or magnify the 

 image on the cornea. In the Galilean telescope the convex lens 

 magnifies the erect image which it forms on the concave eye- 

 glass, the use of which, Ity regulating the sphere of concavity, is to 

 obviate the colours produced by the sphere of convexity: hence 

 an achromatic and magnified corneal image is formed. 



I shall here notice a difliculty which Dr. Barro'.v and :dl other 

 opticians have failed to clear up, particularly noticed by the Bi- 

 shop of Cloyne : " Let an object be placed beyond the focus of 

 a convex lens, and if the eye be close to the lens it will a])peai- 

 D J con! used, 



