[ 139 ] 
V r . The Stress-strain Properties of Nitro-cellulose and the Law of its 
Optical Behaviour. 
By Prof. E. G. Coker, F.R.S., and K. C. Chakko, M.Sc., University College , 
London. 
Received September 22, 1919,—Read January 22, 1920. 
The physical characteristics of transparent bodies capable of resisting stress have 
been the subject of much investigation, and in particular the properties of various 
glasses^ have been studied with much thoroughness since these latter have an 
extensive use both for commercial and scientific purposes. 
In recent years many new forms of optical materials have found an industrial use, 
and especially nitro-cellulose compounds, which are valuable in cases where glass is 
not suitable. 
The mechanical and optical properties of such bodies have not, so far, been examined 
in very great detail, and the present paper describes some experimental evidence 
which has been obtained and which it is hoped to extend as opportunity occurs, since 
this has an important bearing on the study of stress problems arising in engineering 
practice. 
The principal matters which are examined in the present communication are the 
mechanical properties of nitro-cellulose under pure tensile and bending stresses and 
the laws of its optical behaviour under these kinds of stress. In the course of the 
experimental study a considerable number of specimens have been examined, all of 
which are the manufacture of the British Xylonite Company. 
The salient features of the material are its great flexibility and toughness, and the 
ease with which it can be drilled, turned or machined. By suitable adjustment of 
the condition of nitration of the body the hardness of the material can be varied 
through a considerable range, but owing to the difficulties created by the stress of 
war it has not been possible to make this investigation cover materials possessing a 
great range of hardness, and in fact all the specimens taken were originally selected 
* L. N. G. FlLON, “ On the Variation with the Wave-length of the Double Refraction in Strained 
Glass,” ‘Camb. Phil. Soc. Proc.,’ vol. XI., Part VI.; vol. XII., Part I.; and vol. XII., Part V.; see also 
‘ Phil. Trans.,’ A, vol. 207, and ‘ Roy. Soc. Proc.,’ A, vols. 79 and 89. F. Pockels, “ Uber die Aenderung 
des Optischen Verhaltens verschiedener Glaser durch elastische Deformation,” ‘Anna! d. Physik,’ 1902, 
and F. D. Adams and E. G. Coker, “The Cubical Compressibility of Rocks,” ‘Trans. Carnegie Institute.’ 
VOL, CCXXI,-A 586. X [Published October IS, 1920. 
