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LXIII. Remarks on some curious Facts respecting Vision de- 

 scribed in the Lond, and Edinb, Phil, Mag, for 1834. By 

 Lewis Tonna. 



To the Editors of the Philosophical Magazine, 



Gentlemen, 



T HAVE just seen a letter from K. relating to " some cu- 

 -■- rious facts respecting vision," which appeared in your 

 Journal for November 1834. I will not trespass further on 

 your pages than to state that for the last six years a similar 

 difference in power of vision has existed in my eyes. By al- 

 tering the axis of vision of the two eyes, and thus producing 

 a double image of any object, the image offered by the left 

 eye is even less than half the size of the one presented by the 

 right eye. There is also a slight indistinctness in the image, 

 independently of the reduction in size. This affection came on 

 gradually, and was not produced by any disease, either con- 

 stitutional or local, that I am aware of. Whether I am right 

 in attributing this fact to an undue diminution of convexity 

 in the left eye, similar to that habitual to old age, I know 

 not. 



In a sound state of vision, it is doubtless by an habitual 

 and imperceptible exertion of the brain that the images of- 

 fered by the two eyes are made to coincide and produce the 

 perception of one single image. 



In my case the impossibility of producing a coincidence of 

 images of different magnitudes causes a general indistinctness 

 of vision, and I can see objects clearer and better defined with 

 the right eye alone than with both eyes ; but on applying a 

 convex lens of great power (which I always use,) to the left 

 eye, the distinctness is restored, and all colours immediately 

 become more vivid. I should be glad to hear the opinion of 

 persons better able than myself to judge of this phaenomenon, 

 and whether the use of a lens is judicious. The expansion and 

 contraction of the pupil on sudden exposure to changes of 

 intensity of light, is more sluggish in the left or diseased eye 

 than in the right one. This is the only external difference. 

 I am. Gentlemen, yours, &c., 



United Service Museum, London, LewiS Tonna. 



April 16, 1835. 



\* We may now state that the author of the paper signed "K." above 

 referred to, was the late lamented Capt. Kater, in whom the disease of 

 vision it describes seems to have been the precursor of death. It was 

 probably almost the last contribution to science of that distinguished 

 natural philosopher. — Edit. 



Third Series, Vol. 6. No. 36. June 1835. 3 G 



