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X. An Experimental Determination of the Values of the Velocities of Normal 
Propagation of Plane Waves in different directions in a Biaxal Crystal , and a 
Comparison of the Results with Theory. 
By B.. T. Glazebrook, B.A., Fellow of Trinity College, Cambridge. 
Communicated by J. Clerk Maxwell, M.A., F.R.S. 
Received June 19,—Read June 20, 1878. 
PART I. 
Preliminary. —Professor Stokes’s Report to the British Association, 1862, with Outline 
of the Method. 
Section I.— Review of Previous Experiments and Criticism of Fresnel’s. 
In his report to the British Association in 1862, Professor Stokes says : “ The exact¬ 
ness of the spheroidal form, assigned by Huyghens to the sheet of the wave sm-face 
within Iceland spar, does not seem to have been tested to the same degree of rigour 
as the ordinary refraction of the ordinary ray; for the methods applied by Wollaston 
(Phil. Trans., 1802, p. 381) and Malus (Mem. cle lTnstitut Sav. Etran., tom. ii., p. 303, 
1811) for observing the extraordinary refraction can hardly bear comparison for 
exactness with the method of prismatic refraction adopted for the ordinary ray; and 
observations on the absolute velocities of propagation in different directions within 
biaxal crystals are almost wholly wanting. 
“ This has long been recognized as a desideratum, and it has been suggested to 
employ for the purpose the displacement of fringes of interference. 
“ It seems to me that a slight modification of the ordinary method of prismatic 
refraction would be more convenient and exact. 
“ Let the crystal to be examined be cut, unless natural faces or planes of cleavage 
answer the purpose, so as to have two planes inclined at an angle suitable for the 
measure of refractions; there being at least two natural faces or cleavage planes left 
undestroyed, so as to permit of an exact measure of the directions of any artificial 
faces. The prism thus formed having been mounted as usual and placed in any 
azimuth, let the angle of incidence or emergence (according as the prism remains fixed 
or turns w r ith the telescope) be measured by observing the light reflected from the 
surface, and likewise the deviation for several standard-fixed lines in the spectrum; 
each observation furnishes us with an angle of incidence and the corresponding angle 
