Load on test piece 
OF NITEO-CELLULOSE AND THE LAW OF ITS OPTICAL BEHAVIOUR. 
143 
for some time, but in the experiments described here the flat sides are untouched, and 
the cut edges are unpolished although quite smooth. 
Optical Properties .—There is considerable colour when an over-strained specimen 
is examined under no load in the polariscope. In the parallel part the colour is very- 
uniform, and by comparing this with a previously unstrained specimen under load it 
appears that the permanent colour indicates a complex state of stress, since it could 
not be completely neutralised by a comparison tension piece, nor by bending the 
strained bar itself. 
Ib. 
It may also be noted that nitro-cellulose with good optical properties is not 
apparently procurable, above \ inch thick, and it is difficult therefore to conform to 
the laws of similarity for the test specimens used in this investigation. 
In later experiments the stress-strain properties of the material are examined both 
below and above the elastic limit, and the values of Young’s modulus and Poisson’s 
ratio are measured for a number of specimens of different thickness and varying age. 
Especial attention is also directed to test the validit}’ of the stress-optical law of this 
material since this is a matter of fundamental importance and little attention has so 
far been given to it. 
