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VII. An Experimental Investigation into the Form of the Wave Surface of Quartz. 
By James C. Me Conner, B.A. 
Communicated by B. T. Glazebrook, M.A., F.R.S. 
Received November 9,—Read December 17, 1885. 
I.—Introductory Kemarks. 
About two years ago I read a paper before the Cambridge Philosophical Society 
describing some measurements of the “ dark rings in quartz. The present paper 
contains an account of similar measurements made with greatly improved apparatus, 
and extending over a much larger field. These “ dark rings ” supply a delicate 
method of determining the retardation of the extraordinary wave behind the ordinary 
in the crystal and consequently the separation between the two sheets at various 
points of the wave-surface. 
In quartz the wave-surface may be nearly represented by a prolate spheroid sur¬ 
rounded by a sphere passing through the extremities of its axis. The spheroid is 
slightly flattened at the extremity of its axis and the sphere slightly bulged, so that 
the two no longer touch. The distance between the two at the extremity of the 
axis we know very accurately from observations on the rotatory power, while the 
two radii at the equator of the wave-surface are known from observations with the 
spectrometer. 
In all the theories on the subject these three constants are assumed ; so the 
theoretical surfaces are made to coincide with the true one in the equatorial section 
and to give the true distance between the two sheets at the extremity of the axis. 
At intermediate points their correspondence with the true surface lies open to the 
test of experiment. My former observations were confined to the regions near the 
axis, as it is only there that the bulging and flattening are at all conspicuous. But 
the increased facilities oi a new arrangement for measuring the rings led me to 
extend my observations over the whole surface, so that I have now determined the 
gradually increasing distance between the two sheets from the axis to the equator. 
This was the more necessary because the known values of the radii of the equa¬ 
torial section, or, in other words, of the principal wave-velocities, are not sufficiently 
accurate to give a good measure of their difference. And it is upon this difference 
that the theoretical distance between the two sheets, even in the region near the 
axis, to a great extent depends. 
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