TRANSACTIONS OF SECTION A. 389 
allowance is made for this fact, it becomes necessary to subtract from the 
c 2r NI Pir 
quantity ne'®e an amount equal to ae” so that the resultant conductivity 
ro 
4 ne?rAv : , : : F 
is o= 5597 It is quite an accident that the two corrections discussed act 
a 
in opposite directions. If they had been in the same direction the effect would 
have been even greater. 
MONDAY, SEPTEMBER 15. 
DrParRTMENT oF GENERAL Puysics. 
Joint Discussion with Section G on the Investigation of Complex Stress 
Distribution 
The Construction of Large Polurising Apparatus, &e. 
By Professor E. G. Corer. 
Large polariscopes for directly viewing objects in plane and circularly 
polarised light may be constructed by methods described in a former paper,! but 
for quantitative work and for projection on a screen it is more convenient to 
adopt an arrangement in which a beam of light from an arc-lamp is rendered 
parallel by a iens, and is afterwards polarised by reflection from black glass, or 
by refraction through a series of clear glass plates. ‘The analyser may be of the 
same construction on a similar scale, and the addition of quarter-wave plates 
allows an obiect to be viewed free from the black cross-effect. 
In this manner a parallel beam of light can be obtained capable of showing 
objects several inches in diameter. The field of view may be still further 
increased by producing a diverging polarised beam, and interposing a converging 
lens between the object and the analyser. 
It is, however, essential that the converging lens shall be quite free from 
internal stress to avoid distortion of the optical effect. 
An arrangement of two plano-convex lenses, one on each side of the object, has 
also been proposed by Mesnager 2 for the purpose of increasing the field of view 
with the usual arrangement of Nicol’s prisms. 
The following Papers were then read :— 
1. A Theory of Luminescence and the Relation belween Luminescence 
and Pure Temperature Radiation. By Professor BK. PRincsnem. 
With practical certainty we can say : every phenomenon where Kirchhofi’s law is 
found to hold is a case of pure temperature radiation, every one where Kirchhoft’s law 
is not fulfilled is a phenomenon of luminescence. Now where the laws of black radia- 
tion are known we are able to answer with all precision the question whether Kirch- 
hoff’s law holds or not ina given case. The experiments of several authors on metallic 
vapours glowing in a Bunsen flame have shown that the black temperature is the same 
for the different spectral lines observed in the same flame. So it is very probable 
that Kirchhoft’s law holds in this case. For an exact proof, however, it needs still to be 
demonstrated that the black temperature is identical with the true temperature of the 
gas. This proof has been given by two sets of experiments made in the physical 
laboratory of Breslau, first by Mr. Gibson for thallium vapour glowing in an evacuated 
1 “The Design and Construction of Large Polariscopes,’ by Professors E. G. 
Coker and 8. P. Thompson, 7’'he Proceedings of the Optical Convention, 1912. 
2 *On the Measurement of Internal Tension in Solid Bodies and their-Appli- 
cation,’ by A. Mesnager, International Association for T'esting Materials, Buda- 
pest Congress, 1901. 
