260 Jyr. Reades Experiments for [April, 



Article III. 



Experiments for a neio Theory of Vision. By Dr. Joseph Reade. 



(To Dr. Thomson.) 



SIR, Cork, Dec. I, \8\9. 



Ku\^ia'Ta lisii. Xcn. 



Pkrhaps no subject in natural philosophy has more engaged 

 the attention of the learned, nor claimed mofe interest, than that 

 concerning the proper seat of vision. For 200 years the retinal 

 theory has been maintained, and its difficulties, if not absurdities, 

 softened down by the learned ingenuity of mathematicians and 

 metaphysicians, well aware that to overturn a theory so univer- 

 sally adopted, and stamped with the seal of antiquity, requires a 

 number of well regulated experiments and legitimate deductions, 

 I now commit my endeavours to the candour of the intelligent 

 reader. 



Experiment 1. — Having often remarked, when examining the 

 eyes of patients, that surroundmg objects, such as a lighted 

 candle, &.c- were painted on the transparent cornea in a beauti- 

 ful and minute manner, as on the face of a convex speculum, 

 it occurred to me that the mind might receive impressions or 

 ideas from those erect images ; and I was the more desirous of 

 bringing this interesting suggestion to the test of experiment in 

 consequence of the many difficulties attached to the present 

 system of vision. I now pasted two nari'ow strips of black cloth 

 iu the shape of the letter T, and about three inches in length, on 

 one of the upper panes of a large and well lighted window. I 

 then requested a gentleman Avith a large pupil and good sight to 

 seat himself about four or five feet from the letter, and to fix his 

 eyes steadily on it. Looking into his pupil I perceived the letter 

 T to be minutely yet distinctly painted by reflection. I then took 

 a plano-convex lens in my right hand, such as school l)oys use for 

 burning glasses, and held it close to the pupil. On again looking 

 at the corneal image of the letter T, I perceived it enlarged, or 

 magnified, in all its dimensions, and the spectator said he also 

 perceived it much larger than with the naked eye. On reinoving 

 ihe lens a little further from his eye, I perceived the letter on the 

 pupil not only magnified, but surrounded with colours, and the 

 spectator saw^ the letter large, confused, and surrounded with 

 colours. So far the phenomena of vision answered exactly to 

 the changes of this corneal image. I next rem.oved the lens 

 somewhat further from the eye ; and on looking into it perceived 

 the letter T to be inverted, and the spectator likewise saw it 

 inverted. He now took the lens in his own hand, and placing it 

 at different distances before his eye, 1 was enabled, by means of 



