FOKMAT10N OF NEW HYPOTHESES. 371 



on the necessary process of reasoning. The use of symbolic 

 language (as in chemical equations, &c.), and the presence 

 of the actual substances and apparatus required for the 

 experiments, also afford additional aid, 



Sometimes one man raises an hypothesis and another 

 tests it; thus Cassini suggested the mode of measuring 

 the velocity of light by means of the eclipses of the moons 

 of Jupiter, and Roemer tested it and found it correct. 

 Halley suggested, and succeeding astronomers evolved, the 

 discovery of the method of ascertaining the sun's distance 

 from the earth by means of the transit of Venus. Huy- 

 ghens, in the year 1678, suggested the theory of the exist- 

 ence of an universal ether pervading all bodies and all 

 space, and Young and Fresnel tested it. Also Odling's 

 hypothesis that in ozone the oxygen was condensed one 

 half was partly proved by Brodie in 187 1. 1 



Sometimes, also, one man tries unsuccessfully to 

 imagine the true conception which another afterwards 

 succeeds in conceiving. t Malus sought in vain the for- 

 mula determining the angle at which a transparent surface 

 polarises light ; Sir D. Brewster, with a happy sagacity, 

 discovered the formula to be simply this, that the index 

 of refraction is the tangent of the angle of polarisation.' 2 



1 Brodie, Proceedings of Royal Society, 1872, vol. xx. p. 472 ; Odling, 

 'History of Ozone,' Proceedings of Royal Institution, 1872. 



2 Whewell, Philosophy of tlie Inductive Sciences, vol. ii. p. 542. 



B B 2 



