604 TRANSACTIONS OF SECTION G. 
3. A hidden design can be incorporated in the work and ouly rendered visible 
by means of a screen through which the banker can inspect it. 
4. The new system produces incomparably more beautiful geometrical engrav- 
ing than any of the older methods can accomplish. 
MONDAY, SEPTEMBER 15. 
Joint Discussion with Section A on the Investigation of Complea Stress 
Distribution. 
The following Papers were read :— 
1. The Construction of Large Polarising Apparalus for Use in Lantern 
Projection Work. By Professor EK. G. Coxer, D.Sc.—See p. 389. 
2. The Stress Distributions in Thick Cylinders and Rings, 
By Professor E. G. Coker, D.Sc. 
The difficulty of measuring experimentally the state of stress in thick 
cylinders and rings is well known, and it is important that the means employed 
shall give measurements at a point of a material. 
An optical method of investigation with a transparent material affords a 
comparatively simple and accurate method of experiment for determining the 
directions and magnitudes of the principal stresses at any point of a ring 
constructed from a thick plate. The directions of principal stress at any point 
are determined from the lines of equal inclination obtained with plane polarised 
light, and the difference, p—q, of the principal stresses at the same point is 
obtained from observations of the double refraction effects produced when a 
beam of circularly-polarised light traverses the stressed object. The sum, p+q, 
of the principal stresses is also determined by measurements of the change in 
the thickness of the_ stressed object, using a delicate extensometer capable of 
detecting a change of half-a-millionth of an inch. These latter measurements 
are compared with the changes produced in the thickness of a simple tension or 
compression member of the same material as the object, and in this way the sum 
of the principal stresses is determined without further reference to the physical 
constants of the material. 
These measurements of (a) the sum, () the difference, and (c) the directions 
ot the principal stresses at a point taken as averages over the thickness of the 
plate afford a complete experimental solution of the distribution analogous to 
those obtained mathematically in problems of generalised plane stress. 
The experimental methods are applicable to very complicated cases, and are 
described with reference to the stress distributions in thick cylinders and rings 
of various forms, with and without discontinuities. Special apparatus for 
applying and measuring loads and fluid pressures applied to rings and cylinders 
are also described and illustrated. 
3. The Stream-Line Flow of Solids. By Tuomas Ret. 
The geometrical laws that govern the deformation of plastic materials are 
mainly based on assumptions made in connection with the experimental 
researches of the late M. Tresca. In his experiment on the formation of a jet 
by the plastic flow of lead from an orifice in the end of a cylindrical die-block 
from which the lead, in the form of a cylinder of superposed discs, was forced 
out by means of a piston subjected to pressure in a hydraulic press, the lead 
flows radially towards the axis of the cylinder, displacing a core, which forms 
the jet by the concentric contraction of the cylinder. The author pointed 
out that, although the lead under the particular conditions of the experiment 
