492 Mr Filon, On the variation with the wave-length, etc. 
But here it should be noted that, besides the limited range of 
the observations, which are restricted to three wave-lengths and 
do not go beyond the green 1 2 , Pockels’ method appears open to one 
objection, when the dispersion of double refraction is small. 
He uses a Babinet’s quartz compensator: the observations are 
consequently affected by the dispersion of double refraction in 
quartz: it then hardly seems satisfactory to introduce, as he has 
done, a calculated correction for this, especially when, as in some 
of the cases .he treats, the dispersion of double refraction only 
appears in the final result through this calculated correction. 
Accordingly it seemed of interest to publish the present ex- 
periments, which are free from this source of error, if only as a 
confirmation by an independent method. 
I am at present undertaking a series of observations, of which 
it is hoped eventually to publish an account, with a view to deter- 
mining accurately, if possible, the law of variation of artificial 
double refraction. 
In conclusion I wish to express my thanks to Professor Ewing, 
in whose laboratory this work has been carried out, and also to 
Professor J. J. Thomson for kindly letting me use some of the 
optical apparatus belonging to the Cavendish Laboratory. 
I also wish to gratefully acknowledge the kind help which was 
given me by Mr H. C. Bamage, of St John’s College, in connection 
with the arrangement and identification of the comparison spec- 
trum. 
1 This, I think, accounts for his negative result for glasses of densities between 
2 and 3, the greater part of the dispersion being then in the blue end. 
