On the MOTION of LIGHT. 95 



I SHALL juft add here, that if the aberration of the fixed 

 ftars, obferved in Dr Bradley's manner, as above defcribed, 

 be different with the two telefcopes, as Mr BoscoviCH expeds, 

 or if there be obferved that aberration of terreftrial objeds 

 which he defcribes, light is not accelerated, while refraded to- 

 wards the perpendicular, in the inverfe proportion of the fines 

 of incidence and refradion. But if the two aberrations ihall be 

 found to be the fame, and if no terreflrial aberration fliall be 

 obferved, we have a dired proof of the acceleration of light in 

 the above mentioned proportion, and of its refradion being 

 produced by forces ading perpendicularly to the refrading fur- 

 face, and almoft a demonftration that light confiits of corpuf- 

 cles emitted by the fhining body. There is indeed another 

 way of obferving .the aberration of the fixed ftars, viz. by a. 

 micrometer within the eye-piece of the telefcope. This pro- 

 mifes a different aberration with the two telefcopes. But my 

 thoughts on this fubjedl are not yet ready for the examination 

 of the Society. 



I NOW proceed, in the lafl place, to give the fundamental 

 propofitions refpefling the motion of light, as it is affeded by 

 refrading or reflecting fubflances, which are alfo in motion ; 

 propofitions which will afford an eafy and ready folution of eve- 

 ry queflion which may be propofed. 



There are two ways of eftablifhing the fundamental doc- 

 trines on this fubjed. The firft and beft method is to ftate the 

 leading fadls, or to announce the general phyfical laws as mat- 

 ters of obfervation and experience, and then to give a theory 

 of all the fubordinate phaenomena, in the order of their gene- 

 rality, by fliowing in what manner they are comprehended un- 

 der the general laws already eflablifhed. But, in the prefent 

 cafe, this method cannot be followed. For, in the phsenomena 

 which we obferve, the motion of light is blended with feveral 

 motions which we know to obtain in the refleding and refrad- 

 ing fubftances wliich affed it, and perhaps with many other 



motions 



