COLOURS OF CRYSTALLINE PLATES. 129 



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 expe- 

 riments 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 Plat<>*. 



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 analyzing plates, 

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

 refracting crystal, a portion of the light is reflected, whose quan- 

 tity depends on the position of the interposed crystal. In order 

 to analyze the phenomenon, the crystalline plate may be placed 

 so as to receive the polarized beam perpendicularly, and then 

 turned round in its own 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 interposed. These two 

 positions are those in which the principal 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 inclined 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 white. 

 But M. Arago observed that when the interposed plate is suffi- 



* Essay on Light, pp. Sol, &c. 

 K 



