INTO THE POEM OP THE WAVE-SURFACE OP QUARTZ. 
325 
change of a — b. This has been specially investigated by Dufet who finds the change 
per degree — "000000415. 
The rotatory constant I have discussed at length above, in connexion with the 
small rings. 
The thickness of Plate 1 was found to be 27"65 mins., of Plate 2 19*977 mms., while 
the ratio of Plate 3 to Plate 2 was 1 "01377. 
Plate 2 was cut from the same crystal of quartz as Plate 1. 
The rotation of the plane of polarisation produced by either is related to the 
direction of the light as the rotation to the translation of a right-handed screw. 
Such is usually called left-handed quartz. 
The observations were taken in the Cavendish Laboratory, Cambridge, during the 
months of March and June, 1885. 
V.—Previous Measurements. 
The only previous measurements of the retardation in quartz with which I am 
acquainted are those of Jamin in 1850 (Ann. de Chim., ser. 3, vol. 30, p. 68), those 
of Hecht (Wied. Ann., band 20 [1883], p. 426), and my own (Proc. Camb. Phil. 
Soc., vol. 5 [1883], p. 53). 
Jamin rotated the plate of quartz between two crossed Njcols as I have done, but 
he used parallel light, taking the reading when the field of view appeared darkest. 
Most of the measurements agreed with Cauchy’s theory within one part in thirty, 
though there were a few wider discrepancies. This he considered satisfactory agree¬ 
ment. He obtained similar results from another series of experiments in which the 
axis ratios were also involved. 
Hecht’s method was not very dissimilar to that of Jamin. There is an important 
oversight in his paper. In estimating the probable error of D he makes the tacit 
assumption that a certain quantity he calls <£ is known accurately; while in fact the 
main part of the error results from the inaccuracy of this (/>. Correcting this I obtain 
—using my own notation—for 
22"4' D="1224 (±"0006) 
cf>=7 0 11"4' D="2750 (±’009) 
whence I deduce for the former P 3 = 12"72d; 1"8, for the latter P 3 = 15"82 I h , 57. 
The results obtained in my own earlier paper were vitiated, partly by the addition 
of some corrections founded on an erroneous argument, and partly by the bad faces of 
the plate examined. The argument was as follows. 
The observations were made by bringing the cross wire up to the sensibly dark part 
of the band, first on one side, then on the other. Then I argued that I knew the 
position and breadth of the sensibly dark region of the band, but I did not know that 
by taking the middle of this I should obtain the darkest point. The distance from 
