1821.] Crystallized Bodies on Homogeneous Light, 



COS. <p 



and consequently 



sin. d . sin. 6' = 



2 k t . COS. a 



COS. 6 + COS. ^' 



2 . COS. a 



(cos. 5 -f COS. d') ; 



16J> 



(/) 



To put this to the trial, T took a plate of mica, whose thickness* 

 measured 0-023078 inch, and having adjusted it accurately on a 

 divided apparatus, placed it in an azimuth 45°, and, by the inter- 

 vention of the red glass above-mentioned, observed the maxima 

 and minima of the extraordinary pencil between the poles. As 

 these observations, when repeated, seldom agreed unless within 

 a few minutes, 10 were taken of each maximum and minimum^ 

 The angles of incidence, deduced from a mean of similar obser- 

 vations on each side of the perpendicular, are set down in the 

 second column of the following table, each number in which is, 

 therefore, a mean result of 20 observations. The first column 

 contains the values of //, or the order of the ring observed ; the 

 third, the angles of refraction, to obtain which I used the index 

 1*500, employed by M. Biot.* The fourth and fifth columns 

 contain the values of 5, B% thence computed, and the 6th, values 



of the coeflicient h, deduced from the formula h — 



. sin. 0' 



Table X,^Mica. Thickness = 0-023078 in. 



The last column of this table exhibits the deviations in excess 

 or defect of the values of the quantity //, so computed from the 

 mean of all of them. Their smallness, in comparison with the 

 quantity itself, and their alternations of sign, are evident proofs 



• Recherches sur les Mouvemens des Molecules de la Lumiere, &c. p. 482. He take» 

 it equal to that of glass — " ce qui ne doit pas etre fort eloigne de la verite." I have 

 attempted, without success, to measure its value. What has satisfied M. Biot and Dr- 

 Brewster (for the latter has evidently used this index, or one very near it, Phil. Trans. 

 1818, p. 230) ought to satisfy every one; yet it is fortunate that in the present instance^ 

 a slight variation in the refractive index will produce but a very trifling change in the 

 relativi values of h. 



