Mechanical Philosophy, ,yl 



and observations on the same substance; parti- 

 cularly on the laws of its reflection and refraction. 

 On this subject, indeed, he is placed, by a very 

 adequate judge, among the most eminent ob- 

 servers and discoverers which the eighteenth cen- 

 tury produced." Another species of action of other 

 bodies on the rays of light, producing what philo- 

 sophers have called inflection and de^flection, was 

 f;uggested by our illustrious countryman Dr. Rit- 

 TENiiousE," but was first demonstrated by the in- 

 genious experiments of Mr. Brougham.^ From 

 these and other facts, it appears that light is ope- 

 rated upon by material substances; that it is sub- 

 jected to the laws of attraction, and, of conse- 

 quence, possesses gravity. In the same sphere 

 of experiment and observation may be mentioned 

 Dr. Smith and Mr. Mitchel, of Great-Britain, 

 who made many valuable computations with re- 

 spect to the intenseness, and the best mode of 

 measuring this subtle fluid. The property which 

 various bodies, both animal and vegetable, pos- 

 sess of imbibing and emitting light, has also 

 been investigated 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 few years past; 

 'and the nature and principles of some of these 

 effects ingeniously and satisfactoTily explored. 



n See Priestley's History of Optics, 4to. London, 1772; from which 

 •many of the facts related in this sketch are taken. 



Those who have perused this work need not be informed, that it is a 

 very interesting one; and that the labours of Dr. Priestley, in collecting so 

 many historical facts relative to the science of Optics, together with his own 

 experiments, hints, and inquiries on the subject, entitle him to an honour- 

 able station among those who have deserved well of this science in the 

 •eighteenth century. 



Txansactions American Philosophical Society, vol. ii. 



p Philosophical Transactions for 1796, . . 



