Prof. Powell on the Theory of the Aberration of Light. 427 



observation that the vane at the mast-head took a direction 

 intermediate to that of the wind and of the boat: and the 

 analogy was obvious between the direction in which the wind 

 seemed by this index to come to the boat in motion, and that 

 in which the light from a star seems to come to the earth in 

 motion ; the illustration however is but vague and general. 

 And in the explanations thus adopted generally, that there was 

 still something wanting to give a firm grasp to the apprehen- 

 sion, appears clearly, as well from the loose and wavering con- 

 ceptions betrayed in some of the speculations of that time con- 

 nected with the subject, as in the partial endeavours made by 

 others to fix the view of it more precisely. 



It is perhaps a proof of the defective ideas prevalent on 

 the subject, that on the occasion of the transit of Venus, 1769, 

 Bliss and Hornsby should both have calculated the effect of 

 aberration as accelerating the phases of the transit ; while 

 Prof. Winthorp of Cambridge, United States*, contended by 

 means of a detailed illustration (imagining balls fired at a ship 

 in motion), that the effect ought to be that of retardation, in 

 which he was supported by Dr. Pricef. 



A question somewhat of the same kind was also debated 

 among the French astronomers ; Monnier, Bailly and Cassini, 

 contending that in a transit of a planet it was only necessary 

 to consider the aberration of the sun ; while Lalande intro- 

 duced the difference of the aberrations of the sun and planet: 

 and the former principle was defended by M. Jeaurat, on the 

 ground that the planet was dark, and its existence only shown 

 by the light of the sun surrounding it %. 



The elaborate discussion of Euler§ contributes little perhaps 

 to the precise explanation. He treats the whole subject in a 

 geometrical form; considering the various cases of a luminous 

 body seen by an observer in motion later in time than the 

 truth, even if the observer be at rest, and different in direction 

 if one or both be in motion : but in constructing for the com- 

 position of motions he makes no reference to any further essen- 

 tial conditions. 



Even at a much later period, Dr. Young observed ||, "some 

 mistakes appear to have been made respecting this subject ; 

 they may be avoided by attending to one general principle; 

 that is, when a body moves uniformly forwards, the relative 

 situation of another body, whether quiescent or in motion, 

 appears at all times to be such as it really was at the moment 

 of the emission of the light of the second body ; in other words, 



* Philosophical Transactions, 1770, p. 359. f Ibid, p. 536. 



% Mem. Acad. Paris, 1786, p. 572. § Comment. Petrop. xi. 150. 1739. 



|| Natural Philosophy, ii. 294. 



2 G 2 



