the Aberration of the fixed Stare , See. 6 9 
• % . *• »• 
It has indeed always been taken for granted, that the velo- 
city of the ray which pafles through the center of convexity, 
reprefents the common velocity of all the contemporary light 
of the converging pencil. This may perhaps be reckoned a 
circumfbmce of which we have no proof. But it muff be 
confldered, that if the rays of light, after being varioufly bent 
towards the focus, were no longer to move with tiie fame com- 
mon velocity, the image formed at the focus of Dr. bradley’s 
telefcope, would be elongated in the direction of the aberra- 
tion. Thofe who have attended to this fubjeCt will be at no 
lofs in difeerning the reafon of this. The extent of that 
lengthened image would depend upon the difference of velo- 
city which would obtain among the converging rays, and 
would probably increafe according to the largenefs of the aper- 
ture of the objeCt-glafs. But fuch a phenomenon being con- 
trary to experience, it follows, that the unequal bending of 
the rays does not give them unequal velocities, whilft moving- 
in the fame medium. This is another property with regard to 
the motion of light which may be confi dered as proved experi- 
mentally by Dr. bradley’s obfervations, and which doubtlefs 
* 
would have occurred to him if he had had occafon to trace the 
refraction of a pencil of parallel rays at the objeCt-glafs of his 
telefcope. 
To conclude : in bringing this queffion concerning the ve- 
locity of light to the iffue of an experiment, that fluid would 
doubtlefs be moft proper for the telefcope which abforbs the 
feweft rays, and poflefles the greatefl: refractive denflty, and 
which at the fame time is not liable to generate air-bubbles. 
To compenfite for the unavoidable lofs of light, which by Mr. 
canton and Dr. Priestley’s experiments is found to be con- 
fiderable in fuch cafes, it perhaps may be neceffary to ufe an 
achromatic 
