1820.] « "«'« Theory of Vision. 261 



the corneal image, to tell him what he saw. Having again 

 requested the spectator to fix his eye on the letter, I placed a 

 double concave lens before the pupil, and the letter was 

 immediately dnninished ; he now said he saw it very small. Here 

 I shall beg" leave to remark that these expermients strike at the 

 very first principles as laid down for optical instruments, tor 

 we find by two simple and conclusive experiments, that a convex 

 lens, instead of converging the rays, as first maintained by 

 Maurolycus in his treatise " De Lumine et Umbra, actually 

 and bona fide diverges and magnifies the nnage in all its dimen- 

 sions • and, on the other hand, that a concave glass converges 

 or diminishes the image. The object of this paper being merely 

 to draw the attention of the scientific to my opmions on optics, 

 and particularly on vision, I shall not at present enter more 

 fully into the theory of spectacles, &c. 



kxpcrimmt 2.— Having placed a plano-convex lens at such a 

 distance before the spectator's eve, as to form an inverted image 

 of the letter T on his pupil, 1 placed a double concave lens 

 behind to represent an opera glass, or gallilean telescope. Ihe 

 inverted corneal image immediately became erect, and the spec- 

 tator said he also saw it erect. .14. 

 Experiment 3.— The above experiments were made at 'about 

 four feet from the window ; I now requested the spectator to 

 remove his chair to within a foot of the object, and on placing a 

 convex lens immediately before the eye, the corneal image was 

 considerably magnified : on slowly removing the lens nearer the 

 letter, and further from the eye, the black corneal image began 

 to be surrounded with colours, but did not become inverted, nor 

 did the spectator perceive any change of position: when close to 

 the object, the corneal image appeared better defined and more 

 distinct. I now placed a prism before his eye, and desired him 

 to look through the lower refracting angle; as he was unaccus- 

 tomed to the application of this instrument, he could not regulate 

 it so as to perceive the coloured image of the letter T. I, there- 

 fore, turned the prism until 1 perceived it on the pupd, and then 

 told him exactly what he saw, making a mirror of his eye. Let 

 us now inquire what changes the intervention of a plano-convex, 

 or a concave glass, would make on the letter T brought to a 

 focus on the retina by means of the crystalline and other humours. 

 Having removed the fat and coats fiom the back part of an ox'g 

 eye, as performed by Kepler and Scheiner, and thus laid bare the 

 retina, I placed a lighted candle in front. An inverted image 

 was seen as if floating on the retina. I now placed a plano- 

 convex lens between the candle and the cornea at such a dis- 

 tance as to form an inverted image on the pupil. The retinal 

 image remained inverted. On now placing a double concave 

 lens at a little distance before the convex one, the corneal and 

 inverted image became erect, while the retinal inverted image 



