1820.] a new Theory of Vision. 267 



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

 born bhnd and suddenly restored to sight would suppose every 

 object to touch his eye. All that is accomplished by telescopes 

 and microscopes (according to the retinal theory) is first to make 

 an image of a distant object by 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, compared with the angle which the object itself would sub- 

 tend in the same situation ; this is done by means of an eye-glass 

 which so refracts the pencils of rays that they may afterwards 

 be brought to their several foci by the natural humours of the 

 eye. Now it is evident from the foregoing experiments that this 

 theory is perfectly erroneous, and that a telescope, as shall here- 

 after be more fully shown, does nothing more than diverge the 

 rays, or magnify the image on the cornea. In the gallilean 

 telescope, the convex lens magnifies the erect image which it 

 forms on the concave eye-glass, the use of which, by 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 difficulty which 

 Dr. Barrow and all other opticians have failed to clear up, parti- 

 cularly noticed by the Bishop of Cloyue. " Let an object be 

 placed beyond the focus of a convex lens, and if the eye be close 

 to the lens, it will appear confused, but very near to its true 

 place. If the eye be a little withdrav/n, the confusion will 

 increase, and the object will seem to come nearer ; and when the 

 eye is very near the focus, the confusion will be exceedingly 

 great, and the object will seem to be close to the eye. But in 

 this experiment the eye receives no rays but those that are con- 

 verging; and the point from which they issue is so far from being 

 nearer than the object, that it is beyond it ; notwithstanding 

 which the object is conceived to be much nearer than it is, 

 though no very distinct idea can be formed of its precise dis-, 

 tance." Here Dr. Barrow supposed that when his eye was close 

 to the lens it received none but converging rays ; whereas, on 

 the contrary, they were diverging ; and as he withdrew his eye, 

 the more the erect image was ma2;nified, when magnified beyond 

 the standard of distinct vision it became confused. But when 

 the eye was beyond the focus, the anterior or erect image was 

 lost to the eve, and the two lateral and inverted images coalesc- 

 mg into one, formed an image which was nearer the eye noatmg 

 as if on the posterior surface of the lens. Dr. Barrow, hke a 

 true philosopher, acknowledges himself unable to account for 

 this appearance, finishing his lecture with this observation : 

 " Vobis itaque nodum hunc, utinam feliciore conatu, resolvendum 

 committo." Whether these experiments tend to untying the 

 knot, I leave the reader to determine, and shall not enter on 

 Berkeley's or Barrow's theories of apparent distance in this 

 paper. 



