486 Sir David Brewster on the Cause 



ways of constructing rational polygons, as they may be called, 

 the methods described in Propositions III. and IV. having been 

 chosen merely as the simplest. For example, in a circle whose 

 diameter is 1, let there be an inscribed rational polygon one of 

 whose sides is sin 0. Then if <j> be any angle less than 0, and 

 such that sin <j> and cos <f> are rational, sin <f> and sin (#—</>) will 

 be two of the sides of a new rational inscribed polygon, subtend- 

 ing together the same arc that is subtended by sin in the ori- 

 ginal polygon ; and to the new inscribed polygon will correspond 

 a new circumscribed polygon, which will be rational also. 

 Glasgow University, April 1865. 



LIX. On the Cause and Cure of Cataract. 

 By Sir David Brewster, K.H., F.R.S.* 



MY attention was called to the subject of Cataract, in con- 

 sequence of having, about forty years ago, experienced an 

 incipient attack of that complaint, and studied its progress and 

 cure. 



While engaged in a game at chess with Sir James Hall, who 

 was a very slow player, I amused myself in the intervals with 

 looking at the streams of light which radiated from the flame of 

 a candle in certain positions of the eyelids. In one of these 

 observations I was surprised by a new phenomenon, of which I 

 did not immediately see the cause. The flame of the candle 

 was surrounded with lines of light, of an imperfectly triangular 

 form, some parts of which were deeply tinged with the prismatic 

 colours. Upon going home from the chess club, this optical 

 figure was seen more distinctly round the moon, and of course 

 it appeared, with more or less brightness, round every source of 

 light. 



Having been engaged in examining the structure of the crys- 

 talline lens in animals of all kinds, I soon discovered the cause 

 of the phenomenon which I have described. The laminae of the 

 crystalline lens had separated near its centre, and the separation 

 had extended considerably towards its margin. The albuminous 

 fluid, the liquor Morgagni, which so wonderfully unites into one 

 transparent body, as pure as a drop of water, the mass of toothed 

 fibres which compose the crystalline lens, had not been suffi- 

 ciently supplied ; and if this process of desiccation had continued, 

 the whole laminae of the lens would have separated, and that 

 state of white opacity have been induced which no attempt has 

 ever been made to remove. 



The continuance of this affection of the lens was naturally a 



* From the Transactions of the Royal Society of Edinburgh, vol. xxiv, 

 part 1, Communicated by the Author. 



