1885.] The Form of the Wave Surface of Quartz. 



409 



5. That occasionally, though rarely, the summation of rapidly succeed- 

 ing nervous impulses may be only incompletely effected within the 

 nerve-cells of the spinal cord, or may not occur at all. In these cases 

 results similar to those of Franek and Pitres are obtained. 



A more detailed account of these experiments will shortly be pub- 

 lished in the " Joarnal of Physiology." 



December 17, 1885. 



Professor Gr. G-. STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



Professor Horace Lamb (elected 1884) was admitted into the Society. 

 The following Papers were read : — 



I. " An Experimental Investigation into the Form of the Wave 

 Surface of Quartz." By James C. McConnel, B.A. Com- 

 municated by R. T. Glazebrook, M.A., F.R.S. Received 

 November 9, 1885. 



(Abstract.) 



The paper contains an account of a number of measurements of 

 the well-known " dark rings " of quartz. Each ring is due to one 

 wave being retarded in the quartz behind the other by an integral 

 number of wave-lengths, so the measurements give the directions 

 through the plate of quartz corresponding to a series of known 

 retardations. The relative retardation is, especially in a crystal of 

 weak double-refracting power like quartz, mainly dependent on the 

 distance between the two sheets of the wave surface. Thus my 

 observations really give the separation between the two sheets at 

 various points, and it is in this separation that the peculiarities of 

 quartz are most strongly marked, and the various expressions put- 

 forward by theory most widely divergent. 



With a plate cut at right angles to the axis, I obtained values of 

 the separation from = 4° to = 39° — being the angle between the 

 ordinary wave normal and the axis — and with a plate cut parallel to 

 the axis I obtained values from 0=53° to 0=9O n . 



An obvious danger in this mode of investigating the wave surface 



vol. xxxix. 2 E 



