Remarks on a Paper by Dr Scoresby, 391 
The French Government appointed a Commission in 1839 
to examine M. Courtois’ system, and to report on it to the 
Director-General of Public Works ; the report of the Com- 
mission was favourable in every respect, and dwelt particu- 
larly on the efficiency of the system in affording protection 
against the access of water. 
EpinpureH, 4th December 1841. 
Remarks on a Paper by Dr Scoresby, “‘ On the Colours of the 
Dew Drop,” in the Edin. Phil. Jour. vol. xxxi. p. 50.- Ina 
Letter to the Editor. By Proressor Forzzs. 
EDINBURGH, 21st March 1842. 
My Dear Srr,—In Dr Scoresby’s paper on the Dew Drop, 
in your Journal for July last, a variety of optical phenomena 
are described as new, or at least as unexplained. I wish that 
some other person than myself had pointed out to Dr Scoresby 
and your readers that the facts in question are only particular 
cases of well known results of optical laws. The description 
of the phenomena is so precise, as to leave no doubt as to their 
causes, which, in almost every particular, have been com- 
pletely explained.* I therefore reluctantly trouble you with 
a few words respecting the real state of our knowledge on this 
interesting branch of science. 
The ingenious observations of Dr S. are on two classes of 
phenomena essentially distinct ; though in both cases the re- 
sult is a series of circular coloured bands formed by minute 
drops of water. 
The coloured rings having a radius of 40° or 50° (p. 52 of 
Dr S’s. paper), are true rainbows. The order of colours was 
the same nearly as in the common rainbow ; why they have 
not pure tints will immediately be noticed. 
“ In the globules at sma// angular distances the reverse or- 
der of colours” was observed (p. 52). The origin of these ap- 
pears to be the diffraction of light, and is exactly similar to 
the glories of 6°, 12°, 18°, &e. observed on clouds round the 
1) EE SS SY oP Ses ee an OS Ly | 
* A condensed analysis of this branch of Optics will be found in my Re- 
port on Meteorology, in the Report of the British Association for 1840. 
