56 Experiments on a new Theory of Vision. 



confused, but very near to its true place. If the eye be a little 

 withdrawn the confusion will increase, and the object will seem 

 to come nearer; and when the eye is very near the focus, the con- 

 fusion will be exceedingly great, and the object will seem to be 

 close to the eve. But in this experiment the eye receives no rays 

 but those that are converging, and the point from which they issue 

 is so far from being nearer to the object, that it is beyond it, not- 

 withstanding which the object is conceived to be much nearer than 

 it is, though no very distinct idea can be formed of its precise di- 

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

 to the lens it received none but converging rays, whereas they 

 were diverging, and as he withdrew his eye, the more the erect 

 image was magnified ; 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 eye, and the 

 two lateral and inverted images united into one, forming an 

 image nearer the eye, as if floating on the posterior surface of the 

 lens. Dr. Barrow, like a true philosopher, acknowledges him- 

 self unable to account for this appearance, finishing his lecture 

 with this observation : " Vobis itaque nodum hunc, utinam fe- 

 liciore conatu, resolvendum committo." Whether these experi- 

 ments tend to untying the knot, I leave the reader to determine, 

 and shall not enter on Berkelev's or Barrow's theories of apparent 

 distance in this paper. 



We now come to the rectification of inverted images on the 

 retUia. This according to Scheiner and Kepler is the business 

 of the mind, which, when it perceives an impression on the lower 

 part of the retina, considers it as made by rays jiroceeding from 

 the higher parts of the object tracing the rays back to the pupil, 

 where they cross one another. But this hypothesis, says Dr. 

 Priestley, will hardly i)e deemed satisfactory; and by way of clear- 

 ing up the difficulty he proceeds: This " upper and lower are only 

 relative terms; and as all objects are painted upon the retina in a 

 similar manner, all the upper parts in one direction, and all the 

 lower parts in another, it is bv custom only, founded on experience 

 and the association of ideas, that we learn to distinguish them from 

 one another, so as to direct our eyes or point our hands upv.'ards or 

 downwards as we lipve occasion. If this he the true solution (con- 

 tinues the learned Doctor), it will follow, that if the images of ob- 

 jects had always been painted in a different manner — that is, erect 

 as the objects themselves are — we should have acted as we do now 

 without iieing sensible of the difference, a different association of 

 ideasonly having taken j^lace." Now all this laboured explanation 

 conies to nothing more or less than that we are taught by expe- 

 rience. However, we never find the infant or the brute (both in- 

 capable 



