70 Britain's Heritage of Science 



that opened the road to a much further and more scientific 

 prosecution of the discovery, if it can be called by that 

 name," and finally adopts Coulomb's measurements as con- 

 clusive. It seems, however, to have escaped notice hitherto 

 that Robison in his experiments used what must be con- 

 sidered to be the first absolute electrometer, the electric 

 force being balanced by the action of gravity, and there- 

 fore reducible to its value in terms of dynamical units. 



Robison was a strong adherent of Boscovich, the Italian 

 philosopher, who tried to dispose of the difficulties inherent 

 in the definition of matter by considering atoms to be merely 

 centres of forces without extension. Boscovich had applied 

 his theory to the effects of ponderable matter on the trans- 

 mission of light, and Robison took up this subject and treated 

 it in a paper (Ed. Phil. Trans., Vol. II., 1790), which in 

 many ways is remarkable. Its title, " On the motion of light 

 as affected by refracting and reflecting substances which are in 

 motion," shows that it deals with one of the most puzzling and 

 difficult problems of physics . It was the phenomenon of aberra- 

 tion of fight discovered by Bradley which gave practical im- 

 portance to the subject, and, without entering into details, it 

 deserves to be recorded that Robison had the idea of apply- 

 ing telescopes filled with water to clear up experimentally 

 some of the obscure points, which up to our own times have 

 puzzled mathematicians. This idea was revived and success- 

 fully applied later by Airy, but Robison failed on account 

 of the difficulty of obtaining water that was sufficiently 

 transparent. Although his ideas are now superseded, the 

 paper gives us some idea of the powers of the man of whom 

 Watt wrote : "He was a man of the clearest head and the 

 most science of anybody I have ever known." 



Robison's successor, both in the Chair of Physics and 

 as Secretary of the Royal Society of Edinburgh, was John 

 Playfair (1748-1819), previously Professor of Mathematics, 

 who had taken part in the geological survey connected 

 with the Schehallien experiment of Maskelyne and Robert 

 Hutton. His first work was a book on " Button's Theory 

 of the Earth," which had considerable influence in making 

 James Button's geological theories known and appreciated. 

 His mathematical contribution to science is mainly con- 



