178 REPORT—1885. 
Von Lang holds that the experimental law connecting the rotation 
and the wave length is 
I! 
Rotation = & + DP A 
and this is given by the above expressions if 
e=m+r+ 
fy 
c= m + _ + 
@= rl +5 
0s F a 
N pene 5 of dv_ dw 
o reason is given for assuming the form 4 ie rather than 
3 3 
that selected by MacCullagh, 6? (a-a , which leads to the correct 
dz dy 
relation between the rotation and the wave length without any violent 
supposition as to the form of 6”, such as is made by Von Lang; and, 
though neither theory has any mechanical basis, this fact alone is suffi- 
cient to render MacCullagh’s the more probable, while experiments on 
the size of the rings produced when convergent polarised light is trans- 
mitted through a plate of quartz cut at right angles to the axis agree 
rather better with MacCullagh’s form than with Von Lang’s. 
§ 13. Another theory of double refraction was developed by Lord 
Rayleigh! in 1871. It had been suggested originally by Rankine,? and 
Stokes in his British Association Report referred to it, and showed that in 
its original form it was untenable, The theory is also given by Boussinesq 
in a paper in ‘ Liouville’s Journal,’ 3 which will be considered in full under 
the next section. 
Lord Rayleigh points out the inconsistency already referred to be- 
tween the theories of double refraction and reflexion given by both Green 
and Cauchy, while, as we shall see when considering the polarisation 
phenomena accompanying the reflexion, diffraction, and scattering of 
light, he believes that Neumann and MacCullagh, though consistent, 
were wrong throughout. He then remarks that the analogy of a solid 
moving ina fluid would suggest that the first effect of the matter mole- 
cules in a transparent body would be to alter the apparent density of the 
solid, and that conceivably this alteration might depend on the direction 
of vibration. He supposes that the statical properties of the ether are 
not altered by the presence of the matter, and the equations of motion 
may be written 
d*u __ ap RoR ty 7 
cde de 
p dy _ dp + By% + (17) 
, Ydt? dy | 
d2w 
dp 
Se a pe ke Byv2 
7 ae Sema 
where p is written for Ac, 6 being the dilatation. 
1 Hon. J. W. Strutt, ‘On Double Refraction,’ Phil. Mag. June 1871. 
? Rankine, Phil. Mag. June, 1851. $ See p. 215. 
