HISTORY of the SOCIETY. 29 



of the fixed ftars indicated the proportion between the orbital 

 velocity of the earch, and the velocity of light in the vitreous 

 humour of the eye. This opinion foon led him into various 

 difcuffions, and, in particular, made him fuppofe, that the aber- 

 ration of the fixed ftars, when determined by obfervations 

 made with a telefcope filled with water, would be different from 

 the aberration determined by obfervations made with a com- 

 mon telefcope. Mr WILSON has given an account of fome im- 

 portant and unexpected confequences refulting from thefe fpe- 

 culations, in the Philofophical Tranfactions of London. About 

 the year 1775 or 1776, Mr WILSON began to entertain an opi- 

 nion that the centre of the folar fyftem was in motion, and in 

 1777, communicated to Mr ROBISON, and others, a paper on 

 this fubject. The water-telefcope was, by this time, become 

 familiar to his thoughts ; and it occurred to him, that it might 

 be employed for deciding this queftion, and even for determin- 

 ing the direction and velocity of this motion ; by means of the 

 difference between the obferved aberration of the fixed ftars and 

 the aberration which mould refult from the earth's orbital mo- 

 tion alone. But various objections and difficulties occurred in> 

 the profecution of this attempt, and Mr WILSON foon after- 

 thought of another method. 



IF the earth be carried, with a great velocity, towards a fixed 

 ftar, whofe rays are made to deviate a little by an achromatic 

 prifm, it will follow, that a conftant angle of incidence will 

 give different angles of total deviation, according to the veloci- 

 ty of the motion ; and this difference will be both, real and ap- 

 parent. Therefore, 



i. LET the telefcope of a meridional quadrant be furniflied 

 with a prifm, refracting a few degrees in altitude. Search, by 

 meridional obfervations, for fuch ftars as exhibit altitudes in- 

 confiftent with Dr BRADLY'S aberrations : The differences 

 will indicate an aberration caufed by a motion of the earth, dif- 

 ferent from its orbital motion round the fun. 



2. FURNISH 



