PRESIDENTIAL ADDRESS. 493 
of arches might be made of glass, and the effects of stresses due to loading 
rendered visible in polarised light. 
Brewster carried his investigations further, by the invention of a ‘ chromatic 
teinometer’ for investigating the nature of strains, and consisting of plates or 
bars of glass subjected to flexure in definite ways for comparison with the body 
under stress. 
At a much later date (1841) Neumann developed an elaborate theory for the 
analysis of strain in transparent bodies due to load, unequal temperature, and 
set, while, still later, the youthful genius of Clerk-Maxwell supplied an algebraic 
solution for the stress distribution in any plate subjected to stresses in its own 
lane. 
‘i The early history of the development of this branch of science is, in fact, 
remarkable for notable contributions at long intervals of time, and the almost 
complete disregard by engineers of its practical importance. 
The application of optical investigation to the determination of stress distri- 
bution in engineering structures and machines has, however, been hindered by 
causes which, although apparently insignificant, have been very real obstacles, 
and among these was the absence of a transparent material which could be 
fashioned into shapes suitable for investigating technical problems. It is not 
an easy matter, for example, to construct a glass model of a bridge free from 
internal stress, in the manner suggested by Brewster; and, moreover, glass is 
extremely fragile under load, especially in cases where the stress distribution 
in it varies very much, while the cost of construction is very great. Happily 
there is now no necessity to employ glass for experimental investigation on 
engineering problems, since modern chemistry has supplied artificial bodies, such 
as the nitro-cellulose compounds used for many trade purposes, which have 
optical properties very little inferior to glass, are able to bear great stresses 
without injury, and also are capable of being fashioned with the ease and cer- 
tainty of a wooden model. Photographic processes are also able to reproduce the 
brilliant colour effects caused by stress in transparent materials, so that per- 
manent records can now be made for future reference. 
The construction of polariscopes for examining models on a large scale is 
very essential for technical research, and the great scarcity of Iceland spar of 
sufficient purity and size for use as Nicol’s prisms has caused much attention 
to be paid to the construction of apparatus for producing plane polarised light 
by the aid of sheets of glass. Fortunately this presents little difficulty, and 
although the light is not nearly so well polarised as that obtained from a Nicol’s 
prism it is sufficiently so for the purpose. Large quarter-wave plates of mica 
have also been constructed by my. colleague, Professor Silvanus Thompson, 
F.R.S., for obtaining circularly polarised light, and these have proved suffi- 
ciently exact and exceedingly useful for large models. 
It is of importance to show that the stress distribution revealed by a 
polarised beam of light passing through an elastic transparent material in no 
way differs from that obtained by other means, and evidence is available in 
modern researches, especially by Filon, that the experimental results obtained 
with glass agree with those of the theory of elasticity, while a satisfactory 
agreement of a similar kind has also been obtained with nitro-cellulose com- 
pounds, although not in so complete and direct a manner. Such an agreement 
may be expected on theoretical grounds, since the values of the elastic constants 
do not affect the fundamental equations for stresses in a plane, and although for 
three-dimensional stress the effect of the stretch-squeeze ratio causes some 
difference, yet this is usually negligible. 
Most of the physical constants of glass have been determined with very con- 
siderable accuracy, but other transparent substances have so far received little 
attention, and their optical constants are not well known. The stress-strain 
relations of glass and nitro-cellulose have been determined with considerable 
accuracy, and a useful idea of their relation to metals may be gained from the 
values of the stretch-modulus, EF, and the stretch-squeeze ratio, s. 
The accompanying table shows some average values for a few important 
materials, and it is of interest to note that the stretch-squeeze ratios of cast 
iron and plate-glass are very similar, while the values of the stretch modulus 
are nearly as three to two. These two materials also possess other like charac- 
