MR. R. T. GLAZEBROOK ON PLANE WAVES IN A BIAXAL CRYSTAL. 
28.9 
“ Hence we can find the velocity of propagation of the wave, the normal to which 
lies in a plane perpendicular to the faces of the prism, and makes known angles with 
those faces, and hence with the crystallographic axes.” 
In accordance with these suggestions, I undertook a series of observations at the 
Cavendish Laboratory, Cambridge, which I propose in the present paper to describe; I 
also wish to discuss the results arrived at, and to compare them with those deduced 
from Fresnel’s and some of the rival theories of double refraction. 
But previous to this it seems natural to devote some space to the consideration of 
the experiments that have already been made with the same object. 
These we may divide into two classes: (1.) Those which have reference to Iceland 
spar and other uniaxal crystals ; (2.) Those in which biaxal crystals were used. 
Brewster (Thirteenth Report of the British Association) proved conclusively that 
one wave in Iceland spar obeys strictly the ordinary law of refraction. 
Swan (Edinburgh Trans., vol. xvi., p. 375) obtained by direct measurements with 
prisms placed in the position of minimum deviation values of the ordinary refractive 
index, which differ at most by ’00002. 
To verify his construction for the extraordinary ray, Huyghens himself made but 
few experiments, and it was not till 1802 that Wollaston (Phil. Trans., 1802, p. 381) 
attempted to test the theory with any degree of exactness. 
In 1810 Malus (“ Theorie de la Double Refraction,” Paris Mem. des Savants 
Etrangers, tom. ii., p. 303) undertook a series of experiments with the same object in 
view. 
Rather later, Biot undertook the same task, while more recently Rudberg (Pogg. 
Ann., vol. xiv., p. 45) and Mascart have measured the values of the principal indices 
by means of prisms cut parallel to the axis. 
Since 1862 Professor Stokes has applied the method indicated above to prisms of 
Iceland spar, and finds the results of experiment agree with Huyghens’s construction 
with a possible difference of ‘0001 in the values of the refractive index. The details 
of the experiments are as yet unpublished. 
(2.) We must consider next the case of biaxal crystals. Various experimenters have 
determined the values of the principal refractive indices for different crystals. Fresnel 
alone has endeavoured to verify his theory by experiment (‘ QEuvres Completes de 
Fresnel,’ tom. ii., p. 415; second supplement to the “ Memoire sur la Double Refrac¬ 
tion”). The method adopted was to observe the displacement of the fringes of inter¬ 
ference, formed from two parallel slits, produced by introducing two plates of topaz of 
the same thickness cut in different directions from the same crystal into the paths of 
the pencils proceeding from the two slits respectively, and to compare this with the 
displacement calculated on Fresnel’s theory. 
The first set of observations combined with the angle between the optic axes serves 
to determine the principal velocities a, b, c. a being assumed to be unity, let 
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MDCCCLXXIX. 
