Prof. Forbes’s Letter, 
10^15 
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 E). ForbES. 
We regret to find that we have incurred blame on account of some 
expressions in Mr. Potter’s paper on Photometry in our 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 scientificjournal 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, 
