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XT. Double Refraction and Dispersion in Iceland Spar: an Experimental Investiga¬ 
tion, with a comparison with Huyghen’s Construction for the Extraordinary Wave. 
By R. T. Glazebrook, M.A., Fellow of Trinity College, Cambridge. 
Communicated by Professor J. Clerk Maxwell, M.A., F.R.S. 
Received June 12—Read June 19, 1879. 
Section I. 
Preliminary. 
In a paper read before the Royal Society, June 20, 1878, the results of an inves¬ 
tigation into the truth of Fresnel’s theory of double refraction in a biaxal crystal 
were stated. The comparison between theory and experiment was made by a method 
suggested by Professor Stokes (British Association Report, 1862), according to which 
the reciprocal of the velocity of wave propagation was determined by experiment and 
also on Fresnel’s theory. The greatest difference between the two amounted to 
‘0009, and there appeared to be some connexion between the differences and the 
wave length of the light used. In the endeavour to follow up this connexion I 
undertook a series of similar experiments with light of different wave lengths, using 
three lines of the hydrogen spectrum and the sodium line. The extreme smallness 
of the arragonite prisms I had previously worked with led me to use, at first at least, 
Iceland spar, which could be obtained in large pieces with ease, and for which the 
theoretical calculations were greatly more simple. Professor Stokes had already 
made a series of experiments by the same method with this substance (Proceedings 
of the Royal Society, vol. 20, p. 443) and arrived at results confirming Huyghen’s 
construction. The details of his experiments are as yet unpublished, and I venture to 
think it might be useful to have arranged in tabular form a series of results, to serve 
in the future as a test of any theory of double refraction which might be proposed. 
The method of the experiments, as suggested by Professor Stokes (British Association 
Report, 1862), is as follows : A prism is cut from a piece of spar, and the position of 
its faces with reference to the cleavage faces carefully determined. The prism is 
mounted on a spectrometer, and the collimator adjusted so that the rays of a definite 
wave length falling on the prism are parallel, the edge of the prism being parallel to 
the axis of revolution of the reading telescope. The deviation of the light passing 
through the prism in any position is observed, also the position of the image of the 
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