PHENOMENA OF ABSORPTION, AND THE COLOURS OF THIN PLATES. 247 
by perfectly dark bands, while the transmitted light exhibits analogous bands, which 
are much less dark in consequence of the tint being diluted with a portion of white 
light. The coloured bands of the reflected spectrum occupy the same place among 
the fixed lines of the spectrum as the dark bands of the transmitted one ; and if the 
two spectra were superposed they would form a perfect spectrum, whose rays when 
united would form white light. Hence the reflected and the transmitted tints are 
complementary to each other. 
When this analysis is made with a highly magnified spectrum, the numerous lines 
of which are distinctly seen, it forms one of the most splendid experiments in optics. 
The spectrum is crossed throughout its whole extent with alternate dark and coloured 
bands, increasing in number and diminishing in magnitude with the thickness of the 
plate by which the tint is produced. 
If we use a thin film of mica, of such a thickness as polarizes the white of the first 
order, the transmitted spectrum will be crossed by upwards of three hundred dark 
and three hundred luminous bands, thirty-four of each being included between the 
lines C and D of Fraunhofer, a space less than one tenth of the whole spectrum. 
When we use polarized light, and interpose a doubly refracting plate, and subse- 
quently analyse the transmitted beam, the spectrum is crossed with an analogous 
series of bands, which are still more splendid and more perfect than those given by a 
singly refracting film. The bands in the complementary spectra are equally and per- 
fectly dark ; and when the tints are pure as in calcareous spar, the colours are nearly 
identical with those of thin plates. Through the natural faces of a rhomb of calca- 
reous spar about one sixth of an inch thick, I observed in the space C D above men- 
tioned hundreds of the most minute lines almost as sharp and black as those in the 
solar spectrum. 
In the phenomena of periodical colours which we have now described, there are 
three peculiarities which demand our attention. 1. The dark lines change their 
place by inclining the plate which produces them. 2. Two or more lines never coa- 
lesce into one, and one line of the series is never seen without all the rest being 
equally visible. 3. The colours of the luminous bands in the complementary spectra 
are the same as those of the original spectrum when the thin plate is perfectly colour- 
less. In the case of polarized tints this similarity is not general. 
In order to obtain a correct idea of the phenomena of absorption, I shall describe 
those which are exhibited by a solid, a fluid, and a gaseous body, — by the common 
smalt blue glass, by the green sap of vegetables, and by nitrous acid gas. 
Dr. Young has described the smalt blue glass as dividing the spectrum “into seven 
distinct portions.” I have given in the Edinburgh Transactions* rude coloured draw- 
ings of the effect it produces on the spectrum, and Sir John Herschel'I' has repre- 
sented its action in a different manner. Excepting in the single circumstance of the 
spectrum being divided into bands, there appears no analogy whatever between this 
* Vol. ix. p. 439. Plate XXVII. t Ibid. p. 449. Plate XXVIII. 
