60 Mechanical Philosophy. [Chap. I. 



and refraction of light ; beside many other valua- 

 ble, but less important additions to the science of 

 optics. It must be acknowledged that his doc- 

 trines are by no means free from errors and de- 

 fects ; but these are few in comparison of their 

 great merits, and have been chiefly corrected or 

 supplied by the labours of subsequent philoso- 

 phers. 



Since the discoveries of Newton many important 

 additions have been made to our knowledge of the 

 nature and properties of Light. The materiality 

 of this substance, and the great velocity of its mo- 

 tion, were more fully illustrated and confirmed 

 than they had been before, by Dr. Bradley and 

 Mr. Molyneux, in 172?. A few years afterwards 

 M. Bouguer, a celebrated French philosopher, dis- 

 tinguished himself by his experiments and observa- 

 tions on the same substance 5 particularly on the 

 laws of its reflection and refraction. On this sub- 

 Jt^ct, indeed, he is placed, by a very adequate 

 judge, among the most eminent observers and dis- 

 coverers whom the eighteenth century produced*. 

 Another species of action of other bodies on the 

 rays of light, producing what philosophers have 

 called inflection and deflectioUy was suggested by 

 the illustrious Dr. llittenhouse of America f, but 



* Sec Priestley's History of Optics, 4to, London, 17/2', from 

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



They who have perused tliis 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 honourable station among those 

 who have deserved well of tliis science in the eighteenth century. 



t Trmi^actiom American Philosophical SocictJ/, vol. ii. 



