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LI I. An Account of some curious Facts respecting Vision. 



By K. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



[ THINK the following facts, which, I believe, have not been 

 ■■■ before observed, may be sufficiently interesting to find a 

 place in your Journal. 



About six months since I suffered much from nervous head- 

 aches, which terminated in amaurosis of the left eye. 



For some time after the commencement of the disease, I 

 could perceive with the diseased eye, though with some diffi- 

 culty, the letters forming the body of the Philosophical Ma- 

 gazine. 



Most of your readersare probably aware that by an act 

 of volition it is possible to double the image of an object 

 viewed by both eyes. By means of this power I was enabled 

 readily to transfer the letters of a page of your Magazine, seen 

 by the left (the diseased) eye, to the right of the page, and to 

 cause the bottom of a line of the printing seen with that eye, 

 to coincide with the bottom of a line viewed by the right eye, 

 and I could thus compare with considerable accuracy the re- 

 lative dimensions of the letters as seen by the diseased eye, 

 with those viewed by the sound eye. 



To my great surprise 1 found the letters viewed by the 

 diseased eye to be just one half the height of those seen by 

 the right eye. 



The size of the image formed on the retina must be in both 

 eyes the same, or very nearly the same, the focus of the left 

 eye being about nine, and that of the right eye ten inches. 



From this experiment we seem necessarily led to the strange 

 conclusion that it is not merely by the size of the image 

 formed on the retina that we judge of the dimensions of ob- 

 jects, but that our perception of magnitude must be referred 

 to some other source, and that this perception is modified by 

 the state of the retina. 



I have now lost the power of seeing any type smaller than 

 the words " Philosophical Magazine " in your first page, and 

 those words I should not be able to read did I not know them. 

 It may be necessary to remark, that a very strong light is so 

 far from assisting my vision, that in the light of the sun 

 scarcely a trace of these letters is perceptible. 



That type the letters of which are nearly half an inch high 

 I can read, but it appears cloudy and indistinct. A page of 



