518 BIOGRAPHICAL ACCOUNT 



rifled this conjecture ; the irregularities of Jupiter and Saturn 

 have since been fully explained, and are known to arise chiefly 

 from their action on one another, a very small part only being 

 owing to that of the Georgium Sidus, or of any of the other 

 planets. 



The next publication of Professor Robisont, was a paper in 

 the second volume of the same Transactions, " On the Motion 

 " of Light, as affected by Refracting and Reflecting Substan- 

 " ces, which are themselves in Motion *." 



The phenomena of the aberration of the fixed stars are well 

 known to depend on the velocity of the earth's motion combi- 

 ned with the velocity of light ; the quantity of the aberration, 

 when all other things are given, being directly as the first, and 

 inversely as the second. It is not, however, the general or the 

 medium velocity with which light traverses space, but it is the 

 particular velocity with which it traverses the tube of the tele- 

 scope, that determines the quantity of this aberration. Were 

 it possible, therefore, to increase or diminish that velocity, the 

 aberration would be diminished in the first case, and increased 

 in the second. But, according to the principles now generally 

 received in optics, the velocity of light is increased, when it 

 traverses a denser medium, or one in which the refraction is 

 greater ; and therefore were the tube of a telescope to be filled 

 with water instead of air, the aberration would be diminished. 

 Professor Robison, and his friend Mr Wilson, Professor of 

 Astronomy at Glasgow, had speculated much on this subject, 

 and made many attempts to obtain a water telescope, but, hi- 

 therto, without effect. A paper of Boscovich on the same 

 subject, seemed to suggest some new views, that might render 

 the experiment more easy to be made. That philosopher 



maintained, 



* Edinburgh Transactions, vol. ii. p. 83. 



