262 Dr. Readers Experiment a for [April, 



was not in the least changed as to position. Here is a direct 

 experimental proof that even if an inverted image were painted 

 on the retina, that inverted image, not undergoing any chanoe 

 of position by the intervention of the glasses, could not be the 

 image conveyed to the mind. To suppose for one moment that 

 an mverted image on the retina could produce both the idea of 

 inversion and erection would be adding another inconsistency to 

 Kepler's catalogue. In this experiment the changes of the 

 corneal image were accompanied by simultaneous changes in 

 the mind ; therefore, that, and that alone, must have produced 

 the sensations. On placing a glass globe about two inches dia- 

 meter, filled Avith water, opposite the letter T, and then inter- 

 posing a convex lens, the posterior invertedimage was obliterated; 

 the rays of black light not being sufficiently strong, the same 

 thing took place with a concave lens. Dr. Priestley^ who wrote 

 a number of metaphysical works, gravely informs his readers 

 " that the want of an inverted image might produce the sensa- 

 tion of an erect one." With the highest respect for the Doctor's 

 opinions, we might just as readily believe that the want of a 

 man's dinner would get him a supper ! 



There is no inverted Image ever painted on the Retina. — 

 Having removed the fat and coats from the back part of an 

 ox's eye, and thus bared the retina in imitation of Kepler's 

 and Scheiner's experiments, I placed a hghted candle on a table 

 in front ; and on looking through the retina, my eyes being placed 

 beyond the principal focus of the sphere (or rather two segments 

 of one), I certainly did jierceive a beautiful inverted image of the 

 candle, as if floating on the retina. So far the experiment 

 seemed to accord with the retinal theory of vision ; for if the 

 rays were refracted and converged, as represented by optical 

 writers, by means of the cornea, aqueous humour, crystalline 

 lens, and vitreous humour, they should cross nearly in the centre 

 of the eye, and finally paint an inverted image on the retina. 

 However, on approaching my eye nearer to the retina, I per- 

 ceived the inverted image to become large, confused, and when 

 my eye was very close, it opened into two curved and inverted 

 images, which receded laterally ; and at a yet nearer approach, 

 formed into a circle, through the centre of which I perceived a 

 very distinct and erect image of the candle, evidently coming 

 from the anterior surface of the eye, and perfectly distinct from 

 the inverted one, considerably magnified in passing through the 

 humours. Kepler, in placing his eye beyond the focus of the 

 ox's eye, which is notliing more than a simple sphere, saw an 

 inverted image, formed by the junction of the two images painted 

 on his own cornea, which he mistook for one on the retina, as a 

 person looking into a concave mirror thinks that he sees an 

 inverted image in the glass. Here I think it necessary to give 

 » rough sketch of the passage of the ranys through the eye, 



