210 Prof. Forbes on the Refraction and Polarization of Heat, 



and Berard.* In the case of heat accompanying solar light, 

 it was decisively proved, as might have been anticipated ; but 

 in the case of heat from terrestrial, and especially non-lumi- 

 nous sources, though M. Berard considered that he had proved 

 it, he gives no quantitative measures which could enable us to 

 judge of the evidence, nor does it appear that subsequent ex- 

 perimenters have been able to verify the assertion.f 



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 

 progress in acquiring a knowledge of the true nature of heat 

 will be greatly advanced by our previous analogical acquaint- 

 ance 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 laminae at the polarizing angle 

 might likewise be adapted to the instrument. I had previously 

 noticed the large proportion of heat transmitted by thin plates 

 of mica, and I thought of applying bundles of mica-plates 

 placed at the polarizing angle, and so cut from the plate, that 



• Memoires d'Arcueil, torn. iii. 



t See Professor Powell's papers in the Edinburgh Journal of Science, 

 Second Series, vols. vi. and x. 



J The importance of analogies In science has not perhaps been suffi- 

 ciently insisted on by writers on the methods of philosophizing. A clear 

 perception of connexion has been by far the most fertile source of discover}'. 

 That of gravitation itself was only an extended analogy. The undulatory 

 theory of light has been preeminently indebted to the co-ordinate science 

 of acoustics, which afforded to Dr. Young the most plausible basis of his cu- 

 rious and original investigations; and unless 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 penetrating sagacity of 

 M. Fresnel, in Ins prosecution of the subject, has led him to draw from me- 

 chanical and mathematical analogies, accurate representations of laws which 

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

 vellous prediction of the circular polarization of light by two total reflec- 

 tions 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 phaenomena could never have arrived at the result, — the mere ma- 

 thematician 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. 



