On the MOTION of LIGHT. 97 



and reflexions. He fhows, if this be the cafe, that the com- 

 bined forces of all the particles of a body which act at once on 

 a particle of light, compofe a force whofe direction is perpen- 

 dicular to the furface of the body. When, therefore, thefe 

 forces tend toward the body, the light, at its approach to that 

 body, is accelerated and is refracted toward the perpendicular 

 to its furface. But when thefe forces tend from the body, the 

 light is retarded, refracted from the perpendicular, and fome- 

 times refecled with its former velocity, and in an angle equal to 

 the angle of incidence. When thefe confequences are com- 

 pared with obfervation, the mod complete agreement is found 

 to obtain. Hence Sir Isaac Newton deduces, that when 

 light is obferved to be refracted toward the perpendicular, it has 

 really been actuated by forces tending toward the refracting bo- 

 dy, and that it is accelerated. But when the light is refracted 

 from the perpendicular", it has been actuated by forces tending 

 from the body, and is retarded. Alfo, when it is reflected on 

 its approach to a body at an angle equal to the angle of inci- 

 dence, it has been actuated by forces tending from the body, 

 and moves with its former velocity. When thefe conclufions 

 are combined with the former deductions from the hypothecs, 

 and the refult is compared with obfervation, the mod perfect 

 agreement is dill difcovered. For thefe reafons, this hypothefis 

 has acquired great credit, and deferves to be examined on the 

 prefent occafion. 



The other hypothefis is that of Mr Huyghens and DrHooKE. 

 Thefe gentlemen fuppofe that, as hearing is produced by 

 means of the tremulous motion of elaftic air, which affects the 

 ear, fo virion is produced by the tremulous motion of elaftic 

 light, which affects the eye. This hypothefis was announced 

 and applied to the explanation of phenomena in very general 

 terms, and did not, for a long while, much engage the atten- 

 tion of the learned. The celebrated mathematician Mr Euler 

 has lately brought it into credit, having made fome alterations 



Vol. II. n in 



