214 Sir D. Brewster on Colours in Iridescent Agate. 
the colourless image, and produced by a part of the stripe 
of agate which had a different structure from the part of it 
which gave the spectrum of 28° *. 
Having removed this difficulty I submitted a variety of 
agates to the microscope, and I found some which gave very 
faint diffraction spectra exceedingly close to the colourless 
image, and in those cases J could distinctly see the edges of the 
thin veins of which that part of the agate was composed, the 
number of these veins in an inch corresponding with the di- 
stance of the diffraction spectra. This result encouraged me 
to examine the beautiful specimen represented in fig. 2, plate v. 
of the paper already referred to, in which the part that 
produced the spectra exhibited no other difference from the 
part which did not produce them, than that of having a coarser 
rippled structure. I employed for this purpose a very fine 
achromatic microscope made by Messrs. A. Ross and Co., and 
after an accurate adjustment of the illuminating apparatus, I 
succeeded in discovering that the whole portion of the agate 
which produced the prismatic spectra consisted of veins so 
exceedingly minute that seventeen thousand of them would be 
required to make an inch. If, using Fraunhofer’s letters, we 
call the thickness of a vein 6, the interval between the veins y, 
and é + y = «, thene = g3i5 of an inch; and as 3 is nearly 
equal to y we have § = ;74,dths part of an inch. In other 
specimens I have obtained the following results :— 
Values of < Thickness of each vein or 3. 
1 1 ; 
—— ofan inch. —— inch. 
va, vhaea of an inch 
1 1 
11070 22140 
1 1 
22960 : 45920 
1 1 
25420 50840 
nm 1 i" 1 
27880 55760 
As it is only in the first of these specimens that I have yet 
been able to discover the veined structure, we may consider 
these iridescent agates, when cut into thin plates, so that the 
veins are perpendicular to the two parallel faces, as affording 
the most difficult tests in microscopical observations. 
In diffraction experiments this property of the agate may 
* In fig. 1, plate v. of the Phil. Trans, 1814, A B is the stripe which pro- 
duced these two spectra, the one of 28° being produced by the part 
mopn, and the other of 31° by the part ow p. 
