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 expects, 

 or if there be obferved that aberration of terreftrial objects 

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

 wards the perpendicular, in the inverfe proportion of the fines 

 of incidence and refraction. But if the two aberrations ihallbe 

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

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

 the above mentioned proportion, and of its refraction being 

 produced by forces acting perpendicularly to the refracting fur- 

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

 cles emitted by the mining 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 fubject are not yet ready for the examination 

 of the Society. 



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

 propofitions refpecting the motion of light, as it is affected by 

 refracting or reflecting fubftances, which are alfo in motion ; 

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

 ry queftion which may be propofed. 



There are two ways of eftablifhing the fundamental doc- 

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

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

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

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

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

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

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

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

 motions which we know to obtain in the reflecting and refract- 

 ing fubftances which affect it, and perhaps with many other 



motions 



