104) Prof. Forbes's Letter. 



only in common with perhaps every other person who had 

 attempted it. In this supposition it appears that I was mis- 

 taken. I have since learned from Mr„ Potter himself that he 

 considers the true measure of light as more attainable than 

 that of heat. A proposition so startling, and which is at 

 variance with all that I have ever heard expressed, or should 

 have been disposed to conclude upon the subject, I could not 

 be expected to anticipate, and therefore a collision of opinion, 

 though to be regretted, was unavoidable. 



Before concluding, I have a single observation to offer 

 upon Mr. Warington's interesting communication on Nobili's 

 coloured rings. If Mr. Warington will refer to your Num- 

 ber for July last, page 27, note, (L. & E. Phil. Mag. vol. xv.) 

 he will find the following remark : " The explanation of these 

 colours, by supposing with the philosopher of Reggio (if I un- 

 derstand him aright) that they are produced by thin plates of 

 adhering oxygen gas, is too evidently founded in error to re- 

 quire any notice." I may now add the consideration which seem- 

 ed to me so conclusive, which is not a chemical but an optical 

 one. The colours of thin plates are on all hands admitted to 

 be produced by the interference of the light reflected at their 

 first and second surfaces. In the present case the first surface 

 would be the common boundary of air and oxygen gas, which 

 can neither be considered as a sharp mathematical surface, 

 nor if it could, would there be any appreciable quantity of 

 light reflected from the boundary of substances having 

 scarcely an appreciable difference of refractive power, much less 

 could such intensely vivid colours be the result. This is but 

 one of many palpable oversights in a paper, which, whatever 

 may be its value to artists, seems unworthy of the scientific 

 reputation usually given to Nobili, and in which notwithstand- 

 ing, he speaks with very little respect of the reasonings of 

 Newton and Berzelius. 



I am, my dear Sir, yours very truly, 



Edinburgh, Jan. 21, 1840, James D. ForbES. 



OCj* We regret to find that we have incurred blame on account of some 

 expressions in Mr. Potter's paper on Photometry in onr preceding number, 

 implying a charge of unfairness in the treatment of scientific questions against 

 the Cambridge Philosophical Society, and which are complained of as being, 

 •' under the form of a scientific communication, an irrelevant and most un- 

 just attack upon a public body." We freely admit the justice of the remark 

 of a correspondent, that the editors of a scientificjouinal should avoid giving 

 currency to imputations of this kind; and can only state, that had the na- 

 ture of the charge, and the tone of some other expressions, caught our 

 attention, we should have objected to its admission in the form in which we 

 received it. We can safely appeal to the spirit in which our work has long 

 been conducted in proof of our wish not to occupy its pages with personal 

 imputations, or with the remonstrances to which they necessarily give rise. 

 — Edit. 



