ANALOGY. 297 



instead of being coincident with the direction of vibration, 

 as it would be in the case of transverse sound undulations. 

 Thus the laws of elastic forces appear to be essentially 

 different in application to the luminiferous ether and to 

 ordinary solid bodies \ 



Between light and heat, forms of energy, which at first 

 sight appear so different, a perfect analogy has gradually 

 been established. Not only do rays of light and heat 

 obey exactly the same laws of reflection and refraction, 

 but they are subject to exactly the same laws of absorp- 

 tion and polarization. Wherever a light-ray is deficient 

 in the solar spectrum, a heat-ray is also missing. It is 

 now considered that light is but the influence of heat-rays 

 of certain wave-lengths upon the eye, so that we may in 

 fact cease to distinguish radiant heat and rays of light. 

 Heat in- the radiant condition is, of course, to be distin- 

 guished from the molecular vibration also called heat, 

 and from the potential energy which it produces when 

 absorbed by substances, and rendered latent. 



Use of Analogy in Astronomy. 



We shall be much assisted in gaining a true apprecia- 

 tion of the value of analogy in its feebler degrees, by con- 

 sidering how much it has contributed to the progress of 

 astronomical science. Our point of observation is so fixed 

 with regard to the universe, and our means of examining 

 distant bodies is so restricted, that we are obliged in 

 many cases to be guided by limited and apparently feeble 

 resemblances. In many cases the result has been con- 

 firmed by subsequent direct evidence of the most 

 forcible character. 



While the scientific world was divided in opinion 



i Rankine, 'Philosophical Transactions' (1856), vol. cxlvi. p. 282. 



