vi.] PROFESSORIAL LIFE. 145 



common, with a single exception the variable polariza- 

 bility confirms each other's results. It is for you as a 

 critic to judge how far, if in any case, my right con- 

 clusions were founded on false grounds, for this is the 

 sum of Melloni's captious criticisms.' 



To the ASTRONOMER ROYAL. 



' EDINBURGH, January llth, 1840. 



* Enclosed is a memorandum about some experiments 

 on heat, which are new and unfinished, and I hope you 

 will think important. I cannot help thinking this affec- 

 tion of heat by mechanical surfaces and textures one 

 of the most singular and important yet noticed. For 



ince, is there any way in which we can trace the 



>n of striae on transparent surfaces upon heat or light 

 different from physical interruptions or spare spaces like 

 fine wires ? Has anyone thought of proposing this 

 question in the case of light, viz.: "Is the illumination 

 of a screen by parallel rays of light passing through a 

 grating, always determinable by the area of the interstices 

 compared to the whole area of the grating ? " Can you 

 refer me to any investigation of this ? Is there any 



nd facie absurdity in supposing it should not be so ? 

 If strict? or scratches act really as opaque lines would do, 

 my experiments lead me to think that in the case of heat 

 the interrupting power of the screen depends on X. But 

 1 have not succeeded in getting any wire gauze fine 



igh to test it in that case/ 



To PROFESSOR WHEWBLL. 



'EDINBURGH, February 8th, 1840. 

 . I have lately been making a few preliminary 

 experiments on the form of the elliptic vibrations of 

 heat, in verification of some formulae of Cauchy, which 

 he wrote to me to endeavour to test, and I find no diffi- 

 culty in doing so. The results seem to come out well, 

 and by a graphical process I can readily project the 

 ellipse, find the direction of the greater axis, the excen- 

 iy, &c. You will believe that I truly rejoiced in my 



