52 Experiments on a new Theory of Vision. 



on the side of the basin and under the water ; then immerge the 

 entire lens, and at no distance whatsoever can a focus be ever 

 formed. The circular shadow with its black circumference is 

 perceived, but nothing else. Now surely the refractive power 

 of glass, in proportion to that of Vv'ater, is much greater than that 

 of the crystalline lens in proportion to that of the vitreous hu- 

 mour. Mr. Harris in his Optics, p. 95, says: " It is very diffi- 

 cult, I think, to determine accurately the measure of these refrac- 

 tion ; but, from such experiments as could be made, it has been 

 found that the refractive powers of the aqueous and vitreous hu- 

 mours are each of them much the same with that of common 

 water, and that of the crystalline lens is a little greater : that is, 

 the proportion between the sines of incidence and refraction out 

 of air into the cornea or aqueous humour, is as 4 to 3 : out of 

 the aqueous humour into the crystalline, as 13 to 12, and out of 

 the crystalline humour into the vitreous, as 12 to 13." From ana- 

 logy we are authorized to conclude, that the crystalline lens can 

 never form an inverted image on the retina, and that the lens is 

 placed in the centre of the eye to magnify or diverge the rays, 

 and not to converge and invert the object. Moreover, that the 

 crystalline lens does not produce inverted images on the retina, 

 is shown by what takes place when removed by the operation of 

 ■ depression or extraction. For, if the lens were so essentially neces- 

 sary to vision, its removal must cause blindness. In answer to this 

 objection it has been stated, 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 those complaints, I 

 am ena!)!ed to say that this is by no means the case. In young 

 . patients, the use of convex glasses, although at first of assistance,is 

 .,ultimately unnecessary, if not injurious, as the eye gains strength, 

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

 . as those labouring under short-sightedness. Some time since I 

 -removed a congenital 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. Miss 

 .Jenkins of Bantry writes and reads perfectly well, attending to 

 the business of h^r shop, without the use of spectacles. This 

 Jady came to Cork to consult a London quack, who professed to 

 cure all diseases of the eye that were curable : luckily she did 

 not come under that denomination. Indeed, after the operation 

 in young subjects, I never recommend the use 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 re- 

 moved the lens from the ox's eye, as I have repeatedly done, before 

 the experiment, they would have found that it made not the slightest 



difference 



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