14 REPORT—1840. 
On a new case of Interference. By A. BEL. 
On the Iriscope. By Josern Reap, M.D. 
After noticing the fact of the appearance of colours on plain glass 
which has been washed and then breathed on, while breathing on the 
same glass when perfectly dry yield only a gray vapour, the author 
describes the following experiment. 
“ Having procured a convex plate of black glass, such as that of a 
perspective mirror, I smeared its surface with the top of my finger, 
dipt in a strong solution of Castile soap and distilled water; when dry 
I polished with a coarse towel until no clouds appeared ; I now breathed 
on it through a glass tube about a foot long and one-third of an inch 
diameter, when a beautiful series of concentric and variously coloured 
rings were formed.” 
The author then notices the analogy of the coloured rings thus ob- 
tained with those of Nobili, procured by voltaic agency, and offers 
remarks on the theory of the subject. An instrument constructed for 
the purpose of experiment was exhibited. 
On an Experiment of Interference. By Prof. Powerit. 
The author refers to an experiment of Mr. Potter (in the Lond. 
and Edinb. Journ. of Science, May 1840,) in which, under certain pe- 
culiar conditions, the central stripe appears black, instead of white, as 
required by theory, and in the ordinary form of the experiment. The 
author repeated the experiment in the peculiar way proposed, but still 
found the stripe in question white. His object was to bring it under 
the notice of the Section, and induce others to examine the question, 
especially by micrometrical measurements. 
On a point in the Wave-Theory as applied to Heat. 
By Prof. PowEtt. 
According to M. Cauchy’s theory, the relation between the refract- 
ive index and the wave-length is expressed by the formula 
1 rAx|? wAx | 4 
eae eee ey y | 
and when A is very great, compared with Az, this expression is reduced 
to its limiting value, 
| her 
—=P of, p= TE 
This forms the limit of refraction for rays of all wave-lengths, whe- 
ther of light or of heat. And as the value approaches this limit, consi- 
derable changes in A will correspond only to small changes in p. 
This deduction is obvious, and has been before made. The limit is 
easily determined from Mr. Kelland’s calculation for all the media ex- 
amined by Fraunhofer ; thus e. g. for flint-glass, No. 13, we have 
FortherayB . . . . ¢B = 16277 
For the limit. . . . . go =1°6090 
