Absorption and the Colours of Thin Plates. 211 



identical with those of thin plates. Through the natural faces 

 of a rhomb of calcareous spar about one sixth of an inch thick, 

 I observed in the space C D above mentioned hundreds of 

 the most minute lines almost as sharp and black as those in 

 the solar spectrum. 



In the phaenomena of periodical colours which we have 

 now described, there are three peculiarities which demand 

 our attention. 1. The dark lines change their place by in- 

 clining the plate which produces them. 2. Two or more 

 lines never coalesce 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 colourless. In the case of polarized tints this simi- 

 larity is not general. 



In order to obtain a correct idea of the phaenomena of ab- 

 sorption, I shall describe those which are exhibited by a solid, 

 & 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 drawings of the 

 effect it produces on the spectrum, and Sir John Herschelf 

 has represented 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 phae- 

 nomenon and those of thin plates. The bands diminish in 

 number as the thickness of the plate increases, and their co- 

 lour suffers no other change by inclining the plate but that 

 which arises from the small increase of thickness which the 

 ray traverses. There is one remarkable point of difference 

 between the two classes of phaenomena which requires to be 

 specially attended to. The colours of some of the luminous 

 bands are not the same as those of the spectrum, and therefore 

 the glass has removed certain colours while it has left others 

 of exactly the same refrangibility. The green, for example, 

 is changed into yellow by the removal of blue rays, and in 

 certain glasses a band, almost white, is produced. The co- 

 lours thus removed are said to be absorbed; and by an exten- 

 sive series of experiments with such absorbing substances I 

 have been able to insulate white light in the spectrum, which 

 no prism can decompose, and to establish the existence of three 

 equal and superposed spectra of red, yellow and blue light. 



Analogous phaenomena are exhibited in an alcoholic solu- 



* Vol. jx. p. 439. pi. xxvii. f Ibid. p. 449. pi. xxviii. 



P2 



