428 Sir David Brewster on the Cause 



which were published in the Philosophical Transactions for 1837, 

 the lenses were placed in a glass trough of distilled water, and 

 exposed to polarized light ; and the changes thus produced were 

 indicated by variations in the number and character of the polar- 

 ized rings, and more palpably by the gradual enlargement of the 

 lens. The distilled water passed through the elastic capsule of 

 the lens. The lens increased in size daily, and at the end of 

 several days the capsule burst, leaving the lens in a disorganized 

 state, the outer laminae being reduced to an albuminous pulp by 

 the water admitted through the capsule. 



These experiments have an obvious importance in reference 

 to the cause and cure of the two kinds of cataract to which the 

 human eye is subject. The aqueous humour is in immediate 

 contact with the capsule of the crystalline lens. When, there- 

 fore, the humour contains too little water, the lens has not a 

 sufficient supply of the fluid which keeps its fibres and laminse 

 in optical contact, and hence the laminae separate, and the lens 

 becomes opake and hard. When, on the contrary, the aqueous 

 humour contains too much water, the capsule introduces the 

 excess into the lens, and produces the more dangerous affection 

 of soft cataract, in which it is difficult either to depress or extract 

 the lens. 



In order to cure the first of these kinds of cataract, we must 

 discharge a portion of the aqueous humour, and either supply 

 its place by injecting distilled water, or leave it to nature to sup- 

 ply a more healthy secretion. In order to cure the second kind, 

 we must supply the place of the discharged humour with a solu- 

 tion of albumen ; or, as in the first case, leave to nature the 

 production of a more albuminous secretion. 



These views have received a remarkable confirmation from 

 recent experiments on the artificial production and removal of 

 cataract in the eyes of animals. Dr. Kind, a German physiolo- 

 gist, whom I met at Nice in 1857, informed me that he had pro- 

 duced cataract in guinea-pigs by feeding them with much salt, 

 and that the cataract disappeared when there was no salt in their 

 food. More recently, Dr. Mitchell*, an American physician, 

 produced cataract by injecting syrup into the subcuticular sacs of 

 frogs; and Dr. Richardson f did the same by injecting syrup 

 into the aqueous chamber of the recently dead eye of a sheep. 

 In the experiment of Dr. Mitchell, the cataract was removed 

 from the living eye of the frog by surrounding the animal with 

 water ; and in that of Dr. Richardson, the cataract was removed 

 from the dead eye of the sheep by replacing the syrup with dis- 

 tilled water. 



* American Journal of the Medical Sciences, January 1860. 

 f Medical Times and Gazette, March 31, 1860. 



