HEPOBT ON PHYSICAL OPTICS. 395 



Notwithstanding the important labours of Sir David Brewster, 

 much remains to be done connected with this subject. Sir John 

 Herschel has proposed empirical formulae to represent the in- 

 tensity of the transmitted light as dependent on its direction ; 

 and the results of the formulae present a general accordance 

 with observed facts*. It is much to be desired that these laws 

 should be placed beyond doubt by an extensive series of experi- 

 ments directed to this specific object. Although the laws of 

 absorption by crystallized media are necessarily more compli- 

 cated than those of ordinary media, yet they bear an evident 

 and close relation to the well-known laws of double-refraction, 

 which seems to hold out a clue to their discovery ; and I feel 

 persuaded that it is in the phenomena of dichroism that the 

 physical theory of absorption will first take its rise, and seek its 

 confirmation. 



IV. Colours of Crystalline Plates. 



If a beam of light, polarized by reflexion, be received upon 

 a second reflecting plate at the polarizing angle, it is wholly 

 transmitted when the second plane of incidence is perpendicular 

 to the first. But if between the polarizing and analysing 

 plates, as they are termed, there be interposed a plate of any 

 double-refracting crystal, a portion of the light is reflected, 

 whose quantity depends on the position of the interposed crystal. 

 In order to analyse the phenomenon, the crystalline plate may 

 be placed so as to receive the polarized beam perpendicularly, 

 and then turned round in its OAvn plane. It is then observed 

 that there are two positions of the plate in which the reflected 

 light totally disappears, just as if no crystal had been inter- 

 posed. These two positions are those in which the priiicipal 

 and the perpendicular sections of the crystal coincide with the 

 plane of the first reflexion. When the plate is turned round 

 from either of these positions, the light gradually increases ; 

 and it becomes a maximum when the principal section is in- 

 clined at an angle of 45° to the plane of the first reflexion. These 

 phenomena were observed by Malus. 



The reflected light in these experiments was in all cases 

 ivhite. But M. Arago observed that when the interposed plate 

 is sufiiciently thin, — such as the laminae into which 7nica or 

 sulphate of lime may be readily divided by cleavage, — the most 

 gorgeous colours appear, which vary with every change of incli- 



* Essay on Light, p. 554, &c. 



