389 :: 



ASTRONOMICAL AND NAUTICAL 

 COLLECTIONS. 



i. Elementary View of M^Undulatory Theory of Light, 

 By Mr. Fresnel. 



[Continued from the Number for September.] 



Colours of Crystallized Plates. 



When a pencil of polarised light passes through a rhom- 

 boid of calcarious spar, the principal section of which is 

 parallel to the plane of polarisation, it is well known that 

 the extraordinary image disappears : but it is reproduced 

 when we place before the rhomboid a crystallized j^late, 

 possessed of the property of double refraction, and of which 

 the principal section is neither parallel nor perpendicular to 

 the primitive plane of polarisation : its intensity becomes even 

 equal to that of the ordinary image, when this principal 

 section makes an angle of 45° with the primitive plane. 

 In this case, as well as in the others, both the images are 

 white, if the interposed plate is thick enough, that is, if its 

 thickness is not less than one fiftieth of an inch, when it is 

 of rock crystal or sulfate of lime ; but when it is thinner, 

 the two images are tinted with complementary colours, the 

 nature of which varies with the thickness of the plate, though 

 their brightness only is changed when it is turned round in 

 its plane, keeping it always perpendicular to the incident 

 light. 



This brilliant discovery, for which we are indebted to Mr. 

 Arago, has since occupied much of the attention of all the 

 natural philosophers of Europe, and particularly of Mr. 

 BiOT, Dr. Young, and Dr. Brewster, who have done the 

 most in discovering the laws of the phenomena. Mr. Biot 

 first observed that the colours of the crystallized plates fol- 

 lowed, with respect to their thicknesses, the same laws as 

 the colours of the Newtonian rings; that is to say, that the 

 thicknesses of two crystallized plates of the same nature, 

 which exhibited any two tints, were to each other as the 



