490 



results with those elements which M. Mechain obtained by Laplace's 

 method, with those obtained by Legendre himself, and with those 

 ultimately deduced as corrected elements from the latest observations, 

 how near an approximation is obtained by the method here given ; 

 so that the apparent errors seem rather to be those of observation, 

 which, in fact, are not susceptible of great accuracy even with the 

 best instruments, and with the greatest care, on account of the haze 

 or coma with which these bodies are generally surrounded. 



On the Affections of Light transmitted through crystallized Bodies. By 

 David Brewster, LL.D. F.R.S. Edin. and F.S.A. Edin. In a 

 Letter to Sir Humphry Davy, LL.D. F.R.S. Read December 23, 

 1813. [Phil. Trans. 1814, p. 187.] 



The present experiments, to which the author has been led by dis- 

 covering the singular property of agate described in his former com- 

 munication to the Society, have been attended with results which 

 he considers so extraordinary, that they appear to lead to the very 

 mysteries of physical optics, and exhibit, he says, a series of appear- 

 ances, which far surpass in splendour and variety all other pheno- 

 mena of light. 



This paper treats, first, of the polarizing power of the agate ; se- 

 condly, on that structure of the agate on which its properties depend; 

 thirdly, on peculiar colours exhibited by it ; fourthly, on the depola- 

 rization of light ; and fifthly, on certain elliptic coloured rings pro- 

 duced by obliquely depolarizing crystals. 



With respect to the polarizing power of the agate, Dr. Brewster 

 has before shown that a ray of light transmitted through a slice of 

 laminated agate, cut at right angles to its laminae, may be transmitted 

 through a prism of Iceland spar without being subdivided, being re- 

 fracted ordinarily in one direction, and extraordinarily when the prin- 

 cipal section of the spar is transverse to the laminae of the agate. 

 The author observed at that time a nebulous light that accompanied 

 the bright image of a luminous object seen through the agate, con- 

 sisting of rays that were not similarly affected. He now adds, that 

 this nebulous light is oppositely affected, being refracted like the ex- 

 traordinary rays transmitted through Iceland spar, and accordingly 

 disappearing when the bright image is most discernible, and vice 

 versd. But though the polarization of these rays be different, the 

 refraction of both is the same. 



In order to convey, as accurately as may be, an idea of the struc- 

 ture of the agate having these properties, the author assists his de- 

 scription by delineations of the appearances which the substance itself 

 presents in consequence of the variations in fineness of the laminae, 

 their curvatures, or opacity. Some of the laminae are white, others 

 transparent ; some straight, others variously curved ; and where finest 

 and most transparent, exhibiting an appearance of small waves on a 

 surface of water rippled by a gentle breeze, and depending on small 

 variations of the inclination of the laminae. 



