This paper describes experiments on the birefringence of a strip of plastic material, first when the stretched strip is maintained at constant length and second when it carries a constant load. The results indicate that in neither case is the birefringence proportional to the tension. The data are further examined, and the argument is presented to show that in the constant length experiment the ratio of tensionTat any time after the initial tension,T1, is established should be the same function of the time as is the expression[1−2&mgr;logeLL1]for the constant load experiment. Finally it is shown that the tension ratio data can be represented by a curve of the type,T′T1=Ae−at+Be−bt+Ce−ct.