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XXXVI. On the Cause of the Colours in Iridescent Agate. By 
Sir Davin Brewster, K.H., D.C.L. F.R.S., §& V.P.R.S. 
Edin.* 
+* the Philosophical Transactions for 1813+, I have de- 
scribed the general phenomena of colours produced in iri- 
descent agates of different kinds, but I was not able to dis- 
cover the cause by which these colours were produced. The 
spectra which accompanied the colourless image of a luminous 
body seen through theagate had a decided resemblance to those 
produced by diffraction in grooved surfaces, but as no grooves 
existed on the surface of the agate, as in mother-of-pearl, and 
as no veins could be seen in the interior of the mineral by 
the most powerful microscope which I then possessed, I was 
not entitled to ascribe the colours to an invisible agency. Ina 
subsequent examination { of the coloured images produced by 
certain specimens of calcareous spar, inclosing oppositely cry- 
stallized veins, I was led to suppose, from the observation 
of some similarly placed veins in particular specimens of 
agate, that the colours were those of polarized light as in cal- 
careous spar; but re-examination of the phenomena in new 
specimens of agate afterwards convinced me that this opinion 
was not correct. 
In repeating all my early experiments, with a little more 
experience and knowledge of the subject, I soon perceived that 
the actual phenomena were identical with those of the dif- 
fraction spectra. ‘The coloured spectra in the agate suffered 
no change by increasing or diminishing the thickness of the 
plate. ‘The less refrangible half of the spectrum was greatly 
expanded, and, in some good specimens, I observed the re- 
petition of the spectra ¢hree times at equal intervals, and with 
increasing dispersion. In the specimen represented in plate v. 
fig. 1. of the Philosophical Transactions for 1814§, I had 
obseryed that the second spectrum was only a little further 
distant from the colourless image than the firsé spectrum, 
which was 28° distant from that image. This fact, as it then 
appeared to me, put an end to the supposition that two such 
consecutive spectra could be produced by diffraction; but 
upon re-examining this specimen, I find that, though my ob- 
servation of the fact was correct, yet I was wrong in consider- 
ing it as a second spectrum connected with the jirst spectrum 
of 28°. It was, in reality, a rst spectrum distant 31° from 
* Communicated by the Author. 
+ Phil. Trans. 1813, p. 102, 103; 1814, p. 197-199. 
t Ibid. 1815, p. 287. 
§ Ibid. 1814, p. 198, par. 2. [or Phil. Mag. S. 1. vol, xlii. p. 286, 287 ; 
xliv. p. 267, 268. ] 
