590 SPECIAL METHODS OF DISCOVERY. 



time in the successive revolutions ; from which he inferred 

 the existence of a resisting medium. Uranus still deviates 

 from his tabular place, and the cause yet remains to be 

 discovered.' l The mechanical idea of transverse vibrations 

 of the ether, known as the undulatory theory of light of 

 Young and Fresnel, has enabled us to account with most 

 curious exactness, and to discover by inference, the true 

 causes of many of the most diverse phenomena of light ; 

 the explanation and cause of the phenomena of colours 

 of thin films, striated surfaces, fringed shadows, &c., by 

 interference ; of double refraction by unequal optical elas- 

 ticity in different directions ; of polarisation as being due 

 to transverse vibrations. Various other optical phenomena 

 have been accounted for by this theory facts, some of 

 which Grrimaldi, Newton, and others had observed, but had 

 been unable to reduce to rule or cause. 



It was by studying the phenomena of polarisation of 

 light that Malus, in 1811, was led to discover, by in- 

 ference, the important truth that, whenever we. obtain 

 a polarised ray of light by any means, we also obtain 

 another ray polarised in a direction at right angles to this. 

 It was by means of inference from experimental measure- 

 ments that Brewster discovered the law which in all cases 

 determines the angle at which a reflected beam of light is 

 most completely polarised, and found that ; the index of 

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



Sir J. Herschel, in 1822, by studying the phenomena 

 of substances heated in flames, was led to infer and suggest 

 the hypothesis, that different bodies might be detected by 

 examining the light they emit when highly heated ; and 

 this was tested and found to be true by Fox Talbot in the 

 year 1834. By observing the effect of solar light on salts 



1 Whewell, History of the Inductive Sciences, 3rd edit. vol. ii. p. 224. 

 * Philosophical Transactions oftJw Royal Society, 1815. 



