162 PROF. COKER AND MR. CHAKKO: THE STRESS-STRAIN PROPERTIES, ETC. 
This information lias already been obtained however by observations on precisely 
similar material under direct tension stress, Table II. and fig. 13, and on comparing 
plots of the two sets of data obtained, fig. 16, the agreement is seen to be a remark¬ 
ably close one up to about 3500 lbs. per sq. inch. This agreement is improved if the 
changes in thickness are allowed for, since the corrected curves then lie closer 
together, and strengthen the evidence in favour of the law of optical retardation 
being' 1 an effect of stress and not of strain, and also that it is still a linear function 
much beyond the elastic limit of the material. 
The whole of the evidence, in fact, appears to show that the transparent nitro¬ 
cellulose examined obeys a linear stress optical law which holds up to approximately 
twice the range of the elastic limit of stress ; and that within this range optical 
determinations of stress distribution may be relied upon. 
In conclusion we desire to express our grateful thanks for the help afforded in this 
work by the Department of Scientific and Industrial Research, also for valuable 
suggestions from Prof. Filon, F.R.S., and Prof. Porter, F.R.S., during its progress, 
and for the skilful assistance of Mr. F. H. Withycombe in preparing all the 
experimental apparatus required. 
