496 R. T. Glazebrook on Velocities of Normal [June 20, 



we previously observed in our iron tubes, and described in our last 

 communication, was a reversal of part of these lines, though the latter 

 extend much further towards the blue than we had observed the 

 absorption to extend. In fact, the bright lines extend somewhat more 

 than half the distance between b and F, from 45 to 50 being visible, 

 and placed at nearly equal distances from each other. They also 

 commence close to the b group, i.e., with a wave-length nearly 5,164, 

 but the first two or three lines at that end are not so bright as those 

 which immediately succeed them. The light giving these lines does 

 not extend to more than a short distance from the electrodes, and is 

 generally most conspicuous at the negative electrode. There is a 

 difficulty in consequence of the flickering character of the discharge in 

 getting any accurate measures of them, though they are bright 

 enough, especially at the less refrangible end, to be easily seen. The 

 comparative faintness of the light from the iron tubes appears to us 

 almost sufficient to account for our not having seen the reversed lines 

 so completely as the bright ones ; nevertheless, it is quite in accord- 

 ance with what we in other cases observed, to suppose that some of 

 these lines may be more easily reversed at the temperature of the iron 

 tubes than others. 



XII. "An Experimental Investigation into the Velocities of 

 Normal Propagation of Plane Waves in a Biaxal Crystal, 

 with a Comparison of the Results with Theory." By R. T. 

 Glazebrook, B.A., Fellow of Trinity College, Cambridge. 

 Communicated by J. Clerk Maxwell, M.A., F.R.S. Re- 

 ceived June 19, 1878. 



(Abstract.) 



In his report to the British Association in 1862, Professor Stokes 

 called attention to the desirability of accurate measurements of the 

 velocity of normal propagation of plane waves in a biaxal crystal, with 

 a view to testing by the results Fresnel's theory of double refraction, 

 and suggested then a method to determine this velocity. Let the 

 crystal to be examined be cut into the form of a prism, two or more 

 natural faces being left to determine accurately the position of the cut 

 faces with reference to the axes of elasticity. 



" Let us consider a plane wave of light passing through the crystal 

 prism." 



" Let V be the velocity in air, v in the crystal, let 00' |f have their 

 usual meanings, let i be the angle of the prism, D the deviation of the 

 wave normal after passing through the prism." 



Let us observe the angle of incidence 0, and the deviation D. 



