Editor of the Scientific Memoirs. 381 



tated whether to analyse and expose the partial statements 

 which I am sorry to say that paper contains ; but reflection 

 convinces me that I need not on this occasion deviate from 

 my general rule of avoiding scientific controversy. This letter, 

 which I request you will have the goodness to insert in the 

 Philosophical Magazine, and also in the succeeding Number 

 of the Scientific Memoirs, is intended simply to request those 

 persons who may feel an interest in this branch of science not 

 to take Mr. Melloni's statement of the evidence upon which 

 my conclusions were founded, but to consult the papers 

 themselves, viz. 



On the Refraction and Polarization of Heat. Edinburgh 

 Trans., vol. xiii. and London and Edin. Phil. Mag., 

 vol. vi. 1835. 



Researches on Heat. Second Series. Edin. Trans, xiii, 

 Lond. and Edin. Phil. Mag., vol. xiii. 1838. 



Researches on Heat. Third Series. Edin. Trans, xiv. 

 Lond. and Edin. Phil. Mag., vol. xiii. 1838. 



What has principally determined me to enter into no con- 

 troversy on the matter is this, that however faulty or unsatis- 

 factory my methods of research may have appeared to the 

 Italian philosopher, he has, with but a single exception, con- 

 firmed precisely and unequivocally the results which I was 

 the first to announce. That exception is the fact of the vari- 

 able polarizability of heat from different sources, which is so 

 ingeniously combated in the paper translated in the Scientific 

 Memoirs. On that point I have only to state, that, having 

 examined anew and with every care this curious question, 

 which is considered at length in the third of my papers quoted 

 above, I have not only demonstrated the accuracy of my first 

 assertion, but I have there explained (I believe satisfactorily) 

 the cause of the apparently contrary conclusions obtained by 

 Mr. Melloni. I am, my dear Sir, yours truly, 



James D. Forbes, 



LVIII. On the proper Focus for the Daguerreotype. 

 By John T. Towson. 



To the Editors of the Philosophical Magazine and Journal, 



Gentlemen, 



nPHE universal interest which the discoveries of Daguerreo- 



type and the photographic art have excited, will, I hope, 



excuse my soliciting a space in the pages of your scientific 



Journal, for the purpose of explaining an important fact 



