in modifying Anomalous cases of Vision. 115 



But we are by no means certain, that time produces in eyes, 

 imperfectly constituted in those interesting relations, analo- 

 gous changes ; whether age, in depriving them of the enjoy- 

 ment of the minuter impressions of light, contributes to alter 

 or modify their perceptions of colour, in any other way than 

 a general diminution of its brilliance and power. This, it 

 may be said, can only be determined by correct observations, 

 made at different periods of life ; and perhaps Mr Dalton is 

 the only philosopher capable of affording any experimental 

 information on the subject ; since, in early life, he examined 

 particularly into the peculiarities attendant on his own re- 

 markable perceptions of colour, and, no doubt, has continued 

 to observe, with that cool and discriminating attention for 

 which he is so remarkable, whatever changes may have taken 

 place in his vision, during his useful and brilliant career. 



In the case recorded in the Transactions of the Royal 

 Society of' Edinburgh, the subject of it is not aware of his 

 present perceptions of colour differing in any material de- 

 gree from those which characterized his yearly youth ; but 

 this may possibly have arisen from his being unaccustomed to 

 habits of philosophical observation, and that refined and deli- 

 cate tact by which so many beautiful and interesting truths 

 are discovered. It may not be impossible, however, in the 

 instance of the young man alluded to by the writer of the ar- 

 ticle in question, to discover, by analogous observations and 

 experiments performed at distant intervals of years, whether 

 his perceptions of colour undergo any peculiar change. Time 

 is an element too often neglected in our philosophical investi- 

 gations ; and we are apt sometimes to abandon an inquiry 

 when the materials for its prosecution are only to be obtained 

 by observations made at distant intervals of life. 



I remain, my Dear Sir, 



Yours very faithfully, 



l'i vmouth, May 1st 1826. Gf.okcje Harvey. 



