Sect. VII.] Optics. 61 



was first demonstrated by the ingenious experi- 

 ments of Mr. Brougham*. From tliese and other 

 facts, it appears that light is operated upon by 

 material substances ; that it is subjected to the 

 laws of attraction, and, of consequence, possesses 

 gravity. In the same spliere of experiment and 

 observation may be mentioned Dr. Smith and 

 Mr. Mitchel, of Great Britain, who made many 

 valuable computations with respect to the inten- 

 sity, and the best mode of measuring this subtle 

 fluid. The property which various bodies, botii 

 animal and vegetable, possess of imbibing and 

 emitting light, has also been hivestigated with 

 more success by modern philosophers than during 

 any former period. To which may be added, that 

 a multitude of facts of the most interesting kind^ 

 relating to the effects of light on animal, vegetable, 

 and mineral substances, have been made known 

 within a ^ew years past ; and the nature and prin- 

 ciples of some of these effects ingeniously and sa- 

 tisfactorily explored. 



The theory and laws of Vision have received 

 very great elucidation during the last age. Bishop 

 Berkeley, in his Essay towards a Theory of Vision, 

 published in 1709, solved many difficulties which 

 had attended the subject, and threw much new 

 light upon it. He distinguished more accurately 

 than any who had gone before him, between the 

 immediate objects of sight, and those of the other 

 senses, which become early and insensibly associ- 

 ated with them. He first showed that distance, 

 of itself, cannot be determined immediately by 



* Philosophical Transactions for 1796. 



