Mf. Wilson's propofed Experiment ^ &c. 59 
-and by a very finking agreement with facl, have yet been 
deemed by fome perions as not perfeCUy authentic. His contem- 
porary Leibnitz and others have attempted dcmonfirations of 
the law of refraction from principles very different, and which 
do not lead to the opinion of the acceleration of light in the 
more refracting medium. At prelent it is propoled to point 
out a method ot determining experimentally the law of the 
variation ot the velocity ot light, according to the change of 
the medium. It oblervations fhall fliew this law to be agreea- 
ble to Sir Isaac newton’s conclufions, we fhall then have a 
very firong additional evidence in favour of his principles. If, 
contrary to the mofi probable ifliie of the experiment, fome 
unfufpected law fhould be difcovered, we muft, according to 
the rules of induction laid down by that great matter in phi- 
lolophy, fo far refiriCt our general conclufions, and accommo- 
date our ideas to the real condition of things. 
The method of experiment at prefent alluded to is, that of 
obferving the aberration of the fixed fiars with a telefcope filled 
with a denfe fluid, fuch as water, or any other equally limpid 
and of greater refraCtion, fitted to bring the rays to a focus by 
the furface of the medium oppofed to the objeCt having a pro- 
per degree of convexity. It is enough at this time to fuggeft a 
general notion of the infirument, and we now proceed to ex- 
plain in what manner it can aflifi us in the prefent inquiry. 
Since aberration, taken in its enlarged fenfe, depends on the 
relative velocities of light and of the telefcope, if the rays 
were really to move much fafier or much flower in an unufual 
telefcope of this kind, it feems to follow, that the quantity of 
aberration given in thefe circumfiances, compared with Dr. 
Bradley’s angle, would certainly indicate the new rate of ve- 
locity. Such an inference would certainly be juft, and it is 
J 2 upon 
