and Polarization of Heat. 1 47 



no quantitative measures which could enable us to judge of the 

 evidence, nor does it appear that subsequent experimenters have 

 been able to verify the assertion. * 



32. The importance of the subject will be estimated, when 

 we consider the very definite laws to which the polarization of 

 light is subjected, and the accuracy with which they are repre- 

 sented upon the undulatory hypothesis. If heat, when wholly 

 deprived of light, be subjected to similar modifications, our pro- 

 gress in acquiring a knowledge of the true nature of heat will be 

 greatly advanced by our previous analogical acquaintance with 

 the laws of light, j- 



33. I had been led to make the experiment with tourma- 

 lines, because of the convenience with which all experiments on 

 transmitted heat are made by means of the multiplier. But at 

 the same time it occurred to me, that the transmitted pencil of 

 heat passing through lamina? at the polarizing angle might like- 



* See Professor Powell's papers in the Edinburgh Journal of Science Second 

 Series, vols. vi. and x. 



■f- The importance of analogies in science has not perhaps been sufficiently insist- 

 ed on by writers on the methods of philosophizing. A clear perception of con- 

 nexion has been by far the most fertile source of discovery. That of gravitation 

 itself was only an extended analogy. The undulatory theory of light has been pre- 

 eminently indebted to the co-ordinate science of acoustics, which afforded to Dr 

 Young the most plausible basis of his curious and original investigations; and un- 

 less that science had existed, it may be doubted whether such a speculation would 

 ever have been invented, or, if invented, would have been listened to. The pene- 

 trating sagacity of M. Fiiesnel, in his prosecution of the subject, has led him to 

 draw from mechanical and mathematical analogies, accurate representations of laws 

 which no strict reasoning could have enabled him to arrive at. Of this his marvel- 

 lous prediction of the circular polarization of light by two total reflections in glass, 

 is the most prominent example, a conclusion which no general acuteness could have 

 foreseen, and which was founded on the mere analogy of certain interpretations of 

 imaginary expressions. The mere reasoner about phenomena could never have 

 arrived at the result, — the mere mathematician would have repudiated a deduction 

 founded upon analogy alone. The cause of the long postponement of the discovery 

 of electro-magnetism was the complete apparent breach of analogy between the 

 modes of action of the electric and magnetic forces, and any others previously known. 



T 2 



