of the double refraction in strained glass. 491 
If in the table last given we take the mean of all the values 
of 1/Cp corresponding to wave-lengths lying inside intervals of 
500 tenth-metres, we find 
X 
from 
to 
Mean value 
of (QO-i 
Corrg. value of C in 
sq. cms. per kg. wt. 
6750 
6250 
1060 
2-77 x 10- 6 
6250 
5750 
1063 
2-76 X 10- 6 
5750 
5250 
1056 
2*78 x 10- 6 
5250 
4750 
1048 
2-80 x 10- 6 
4750 
4250 
1027 
2-86 x 10- 6 
4250 
997 
2*94 x 10- G 
It seems therefore probable that, although some hidden source 
of error (possibly a difference in the behaviour of the glass under 
tension and uuder pressure or, again, imperfect homogeneity of the 
glass) may have partly masked this effect in the observations on 
the lower fringes, and exaggerated it in the observations of the 
upper ones, yet the differences obtained are significant. 
Wertheim’s law is therefore not accurately true and the arti- 
ficial double refraction due to strain does exhibit dispersion, the 
difference in the refractive indices being smaller in the red than 
in the violet by about 6 per cent, (taking mean values). 
7. With regard to the law of variation of the coefficient G, 
the present observations are too rough to allow it to be deter- 
mined : the table of mean values given above, however, would 
seem to indicate that G varies far more rapidly towards the violet 
than towards the red. But, having regard to the probable error 
of the observations, no definite conclusion can be drawn. 
Since the above work was done, Herr F. Pockels has published 
an account (see Wied. Ann. 1902, Ser. iv. Vol. vn. p. 745, “ Ueber 
die Aenderung des optischen Verhaltens verschiedener Glaser 
durch elastische Deformation”) of some extremely interesting ex- 
periments on various glasses under compression. He has used 
light of three kinds, namely the Bunsen tiame coloured with Na, 
Li and Tl-salt. 
For heavy glasses Pockels finds a deviation from Wertheim’s 
law of the same kind as the one indicated here, i.e. the coefficient 
G is greater (numerically) for the green and yellow rays than for 
the red. For lighter glasses this dispersion of double refraction 
appears to be insensible. 
