150 PROF. E. G. COKER AND MR. K. C. CHAKKO: THE STRESS-STRAIN PROPERTIES 
plastic region, and it may be worth while at some future time to examine this with 
some care, especially if optical methods are applied to study the distribution of stress 
in purely plastic materials. 
The stress-strain curves obtained for this thin material show a divergence from a 
linear law above 2000 lbs. per sq. inch whether plotted from the direct load or the 
optical stress measurements, but if the direct stress is plotted against its optical 
equivalent there is a definite linear law extending up to at least 4500 lbs. per sq. 
inch, and with only a small divergence at 5000 lbs. per sq. inch. The results in fact 
go to show that the law of retardation is linear as regards stress not only up to the 
elastic limit but actually to at least twice this range, where it is quite impossible for 
the stiain to be linear. This result is shown in all the experiments on good optical 
material, thus in plates •§■ inch thick where the elastic limit appears to be about 
.^50 lbs. pei sq. inch, fig. 8, the corresponding value for fff (i shows no divergence from 
lineality until nearly double this amount, although the curve of fje ceases to he 
linear at about 2500 pounds per sq. inch. 
