77/7; INDUCTIVE OR INVERSE METHOD. 283 



bility. The almost complete coincidence between the 

 spectra of solar, lunar, and planetary light renders it prac- 

 tically certain that the light is all of solar origin, and is 

 reflected from the surfaces of the moon and planets, 

 suffering only slight alteration from the atmospheres of 

 some of the planets. A fresh confirmation of the truth of 

 the Copernican theory is thus furnished. 



A vast probability may be shown to exist that the heat, 

 light, and chemical effects of the sun are due to the same 

 tys, and are so many different manifestations of the same 

 undulations. For a photograph of the spectrum corre- 

 sponds exactly with what the eye observes, allowance being 

 made for the great differences of chemical activity in dif- 

 ferent parts of the spectrum ; and delicate experiments 

 with the thermopile also show that, where there is a dark 

 line, there also the heat of the rays is absent. 



Sir J. Herschel proved the connexion between the di- 

 rection of the oblique faces of symmetrical quartz crystals, 

 and the direction in which the same crystals rotate the 

 plane of the polarisation of light. For if it is found in a 

 second crystal that the relation is the same as in the first, 

 the probability of this happening by chance is ^ ; the 

 probability that in another crystal also the direction 

 would be the same is ^, and so on. The probability that 

 in n + i crystals there would be casual agreement of direc- 

 tion is the w th power of ^, Thus, if in examining fourteen 

 crystals the same relation of the two phenomena is dis- 

 covered in each, the probability that it proceeds from 

 uniform conditions is more than 8000 to i e . Now, since 

 the first observations on this subject were made in 1820, 

 no exceptions have been observed, so that the probability 

 of invariable connexion is incalculably great. 



e 'Edinburgh Review,' No. 185, vol. xcii. July 1850, p. 32 ; Herschel's 

 'Essays,' p. 421; 'Transactions of the Cambridge Philosophical Society/ 

 vol. i. p. 43. 



