of the double refraction in strained glass. 479 
The experiments described below were undertaken with the 
intention of testing the exactness of this law throughout the 
whole of the visible spectrum, Wertheim’s result being derived 
from only three kinds of light, viz. sodium, neutral tint, and red 
glass viewed by transmission, which latter is hardly homogeneous. 
2. The apparatus employed in the experiments was as follows. 
A glass beam AB (Fig. 1), about 30 cms. long, 3'65 cms. 
height and 2*9 cms. thick, was placed on two knife-edges C, D, 
Fig. 1. 
12 cms. apart. Long wooden arms PQ, RS were attached to the 
projecting parts AG, BD of the glass beam by a sort of fish-plate 
arrangement shown in Fig. 1. 
Suspended from these arms by vertical strings hung a long 
wooden beam EF. When the latter was loaded in the middle with 
a weight W, the effect was to apply forces W/ 2 at Q and S and 
therefore a uniform bending moment Wa/2 throughout the part 
of the beam between the knife-edges, a being either distance QC 
or SD. 
Besides this'moment Wa/2 which could be applied at will 
there was always a permanent moment on the beam due to the 
weight of the arms PQ, RS and the projecting parts of the beam 
AB. This permanent moment, however, could be calculated, so 
that the total moment was always known. 
The optical part of the arrangement is shown in plan in Fig. 2. 
Two glass beams such as the one described above were placed side 
by side. Light from a vertical slit T was first rendered parallel 
by a collimating lens L, then passed through a 60° prism P and 
finally focussed by a lens L' , after passing through a polarizing 
Nicol M and through the two beams on a scale s placed on the 
side of the front beam nearest the observer. 
