^4 Sir D. Brevvstei' 07i the Cause, the Preventw??, 



After this stage of perfect development, when there is a 

 marked symmetry both in the relative size and polarizing in- 

 tensities of the four series of sectors, the lens begins to break 

 up. The new negative ring encroaches so much on the two 

 positive ones, which it separates, that the outer one is some- 

 times completely extinguished, while the breadth and tint of 

 the inner sectors are greatly diminished, so that the highest 

 double refraction exists in the newly developed ring. In a 

 day or two this ring also experiences a great change of di- 

 stinctness and intensity, and the lens commonly bursts on the 

 fifth or sixth day, sometimes in the direction of the septa or 

 lines where its fibres have their origin and termination, and 

 sometimes in other directions. 



In order to give a general idea of the cause of these singu- 

 lar changes, I may state that the capsule which incloses the 

 lens is a highly elastic membrane — that it absorbs distilled 

 water abundantly — and that, in consequence of this property, 

 the lens gradually increases in bulk, and becomes more glo- 

 bular, till the capsule bursts with the expansive force of the 

 overgrown lens. That the reaction of the elastic capsule 

 contributes to modify the polarizing structure of the interior 

 mass, cannot admit of a doubt, as it is easy to prove that that 

 structure is altered by mechanical pressure; but I cannot 

 conceive how such a reaction could create a new negative 

 structure between two positive ones, and produce the other 

 phaenomena which I have described. I have been led there- 

 fore to the opinion, that there is in the crystalline lens the 

 germ of the perfect structure, or rather the capability of its 

 being developed by the absorption of the aqueous humour ; 

 that this perfect structure is not produced till the animal 

 frame is completely formed ; and that when it begins to decay 

 the lens changes its density and its focal length, and some- 

 times degenerates into that state which is characterized by 

 hard and soft cataract. 



The results, of which I have now given an exceedingly brief 

 notice, appear to me to afford a satisfactory explanation of 

 those changes in the lens which terminate in cataract, a dis- 

 ease which seems to be more prevalent than in former times. 

 Accidental circumstances have led me to study the progress 

 of this disease in one peculiar case, in which it was arrested 

 and cured; and I am sanguine in the hope that a rational 

 method of preventing, and even of stopping the progress of 

 this alarming disease, before the laminae of the lens have 

 been greatly separated or decomposed, may be deduced from 

 the preceding observations. 



As the experiments, however, and views upon which this 



