406 Sir D. Brewster on the Decomposition and Dispersion 



the eye receives all the blue rays dispersed by the whole length 

 AB of the stratum ; whereas, when we view it in the direction 

 R'C, in the azimuth oi'QP, we only see the tint corresponding 

 to the thickness of the stratum. The tint, however, is, in reality, 

 a maximum in the azimuth of 0^, and gradually diminishes till 

 it ceases in the azimuth of ISO*^, or in the direction CR'. 



If we now immerse in the fluid a plate of colourless glass, 

 whose section is DE, so as to receive the beam ABED, we shall 

 find that there is no peculiar dispersion, as Sir John Herschel 

 observed, either at its surface of incidence or emergence. 

 Hence he concluded that the epipolized beam ABED "is in- 

 capable of undergoing further epipolic dispersion;" and that 

 having thus " lost a property which it originally possessed, it 

 could not therefore be considered qualitatively as the same 

 light." 



Now, in using a condensed beam of light, as we have 

 done, we find that the whole cone ABC, even when two inches 

 long, and with a December sun, disperses the blue light, and 

 the stratum behind the glass plate DE nearly as much as the 

 stratum before it. In fluor-spar and in the other fluids I have 

 mentioned, this is still more strikingly the case*, and hence 

 neither of the conclusions drawn by Sir John Herschel are 

 admissible. 



The following appear to me to be the deductions which the 

 experiments actually authorize : — 



1, A beam of light which has suffered dispersion by the 

 action of a solid or fluid body (that is, an cpipoli::ed beam), is 

 capable of further undergoing epipolic dispersion, provided 

 the thickness of the medium is not so great as to have dispersed 

 all the dispersible rays. 



2. When such a medium is thus rendered incapable of 

 dispersing more light, it is not because it has lost a property 

 which it originally possessed, but because it is deprived of all 

 the dispersible rays which it contained. 



It is no doubt an interesting fact, that a small number of 

 differently coloured rays, constituting blue light by their mix- 

 ture, should possess this pi'operty of being dispersed, while 



* In one of these experiments, a piece of green fluorfrom Alston Moor, 

 when immersed in the quiniferoiis sohition, dispersed a fine wo/e/ i/we light, 

 At \.\\e Aiitrxwce oi three-fourths of an inch from its surface. In another 

 experiment, a beam of light that had been dispersed in the solution of qui- 

 nine, again suffered dispersion at two inctics distance from the surface of a 

 piece of Derbyshire fluor. 



A beam of light that has passed through the esculine solution disperses 

 blue light, but not copiously, when transmitted through the quinine solution; 

 but the beam that has passed through quinine is copiously dispersed when 

 transmitted through esculine. 



