SB4 Dr. Readers Experiments for [April, 



lesis can never form inverted images on the retina, and that the 

 lens is placed in the centre of the eye to magnify or diverge the 

 rays, and not to invert the object. Moreover, that the crystal- 

 line lens does not produce inverted images on the retina is shown 

 by what takes place when removed by the operation of depres- 

 sion or extraction. For if the lens were so essentially necessary 

 to vision, its removal must cause blindness. In answer it has 

 been Sriid, that after the operation, the patient is obliged to use 

 convex glasses, or spectacles, to supply the place of the lens. 

 From many years' practice in these complaints,! am enabled to 

 say, that this is by no means the case. In young patients, the 

 suse of convex glasses, although at first of assistance, is ultimately 

 unnecessary, if not injurious, for as the eye gains strength, they are 

 enabled to see all objects at a limited distance fully as well as 

 those labouring under short-sightedness. Some time since I 

 removed a congenita! cataract from the right eye of Mary Skil- 

 lington, aged 19. After the operation she never wore a glass, 

 and can now see to thread a needle ; she also sees perfectly 

 well at different distances to the extent of 200 feet and upwards. 

 jVIiss Jenkins, of Bantry, writes and reads perfectly well, and 

 attends to the business of her shop without the use of spectacles. 

 This Lady came to Cork to consult a London quack, who pro- 

 fessed to cure all diseases of the eye that were curable : luckily 

 for this patient she did not come under the denomination. Indeed 

 after the operation in young subjects, I never recommend the 

 «se of a convex glass. In those patients wanting the crystalline 

 lens, the rays cannot come to a focus on the retina; yet had 

 Kepler and Scheiner removed the lens from the ox's eye, as I 

 liave repeatedly done, they would have found that it made not 

 the slightest difference in the inverted image, which they con- 

 ceived to float on the retina. Neither could the crystalline jump 

 backwards and forwards to accommodate the eye to the object 

 at different distances. Indeed I cannot conceive the cause of 

 this jumping of the lens. If the distance of the object be ascer- 

 tained, and consequently the object seen before the lens, with 

 its thousands and tens of thousands of muscles, begins to jump, 

 what occasion is there for that movement? But if the jump take 

 place before the object be seen, then the extent of the jump can- 

 not be ascertained. Look before you leap should be a maxim 

 •with all metaphysical jumpers. The fact is, that the eye princi- 

 pally judges of different distances by comparing the visible size 

 of the corneal image with the educated sense of the tangible 

 object, intervening objects, strength of colouring, &c. On view- 

 ing a painting, the objects are all equidistant on the canvass; 

 vet we conceive them to be at relative distances. The Supreme 

 ieing, with an invisible hand, paints the images of external 

 objects on the corneal canvass, and the mind conceives them to 

 be at relative distances on the same principle. The following 

 easy experiments may also show that the rays diverge in passing 



