On the MOTION of LIGHT'. 97 



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

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

 a particle of light, compofe a force whofe diredion 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 refradted toward the perpendicular 

 to its furface. But when thefe forces t^ndi from the body, tlie 

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

 times refe&ed with its former velocity, and in an angle equal to 

 the angle of incidence. When thefe confequences are com- 

 pared with obfervation, the moft complete agreement is found 

 to obtain. Hence Sir Isaac Newton deduces, that when 

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

 really been adluated by forces tending toward the refracting bo- 

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

 from the perpendicular, it has been adtuated by forces tending 

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

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

 dence, it has been adluaied by forces tending from the bodj, 

 and moves with its former velocity. When thefe conclufions 

 are combined with the former dedudlions from the hypothefis, 

 and the refult is compared with obfervation, the moft perfedl 

 agreement is ftill 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 Dr Hooke. 

 Thefe gentlemen fuppofe that, as hearing is produced by 

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

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

 light, which afFe<51s the eye. This hypothefis was announced 

 and applied to the explanation of phaenomena 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 



