494 TRANSACTIONS OF SECTION G. 
teristics: they are both very brittle, and possess well-developed crystalline struc- 
ture, so that we may expect the properties of cast iron under stress to be very 
faithfully followed by plate-glass. 
Material E c 
Steel 4 ‘ ; : 5 30,000,000 0°25 
Wroughtiron . . . . 28,000,000 0:28 
Cast iron . : ; 3 4 15,000,000 0:25 
Plate-glass . : , ‘ : 10,500,000 0:23 
Nitro-cellulose . ; ; : 260,000 to 300,000 0:40 
The high values of the stretch modulus for steel and wrought iron are not, 
apparently, approached by any transparent material having similar ductile pro- 
perties, but although nitro-cellulose has a stretch modulus of rather less than 
one-hundredth that of steel, its stress-strain properties are not unlike. In some 
recent experiments with a miniature testing machine fitted with an arrangement 
for recording the stress-strain relations of xylonite throughout the whole range 
of stress up to fracture, the main characteristics of steel appear on a very much 
reduced scale, and give additional confidence that the results of optical experi- 
ments on this material are applicable to metal structures. 
The complete analysis of stress distribution in a plate is not, however, a 
simple matter, and the analysis of Clerk-Maxwell was intended to provide a 
solution based on the properties of the isochromatic and isoclinic lines, coupled 
with the law that the optical effect is proportional to the difference of the 
principal stresses at a point, and to the thickness of the plate. 
A principal stress perpendicular to the bounding planes is assumed to have 
no optical effect ; but since many cases have arisen where there are three principal 
stress components, it seemed desirable to examine such a case experimentally. 
It is a matter of some difficulty to arrange apparatus to stress a specimen in 
the direction of the incident beam, and at the same time observe the optical 
effect free from disturbing causes, since a transparent medium must be inter- 
posed for applying the required load, and this will be subject to stresses which 
may interfere with the optical effect on the specimen. 
Some observations on circular plates clamped at the edges and uniformly 
loaded over one face, showed that the bending stresses produced in the plate 
caused very little optical effect, since the tension and compression stresses 
neutralised one another, while the shear effects also appeared to be practically 
negligible. The only remaining stresses of importance were those caused by the 
clamping plates at the boundary, which produced radial and circumferential 
stresses having circular symmetry, and as the optical effects of these latter dis- 
appeared at a small distance from the edge, a field of view was obtained in which 
the optical effects of load applied perpendicularly to the plate were quite small, 
even when the internal stresses were very great. 
Two circular plates clamped together to enclose a space between them may 
therefore be used as windows for observing the effect of a uniform pressure upon 
a transparent specimen, which latter may be a plate with its faces parallel to 
the end plates closing the chamber. If cubical compression is applied by a fluid, 
the principal stresses in the plane of the plate produce opposing optical effects, 
and any remaining effect is due to perpendicular pressures on the faces. The 
arrangement of experimental apparatus, therefore, took the form of a pair of 
transparent windows separated by an annular disc, and firmly clamped together 
by collars. The central chamber so formed was subjected to pressure of air, or 
other fluid, up to about one thousand pounds per square inch, and afterwards 
the specimen was introduced and the same pressure applied; but no visible 
change of effect could be observed. Finally, the specimen was set in the field 
of view outside the chamber, and pressure again applied by the fluid, but still 
no change was apparent. In all three cases the optical effects produced were 
small, and practically alike, so that the experimental evidence appears to warrant 
the conclusion that a principal stress in the direction of an incident beam of 
