430 SECTIONAL TRANSACTIONS.—A. 
16. Prof. P. Enrenrest.—Remarks on Quantisation. (See p. 508.) 
17. Prof. P. Lanceviy.—The Structure of Atoms and their Magnetic 
Properties. (See p. 510.) 
18. Prof. R. W. Woop, For. Mem. R.S., and Dr. A. ELLETT.— 
The Effects of Weak Magnetic Fields on the Polarisation of 
Resonance Radiation. 
The resonance radiation of mercury vapour in vacuo, at a pressure of about 
0.0001mm., excited by polarised 2536 radiation, is polarised to the extent of 
90 per cent. This polarisation is completely destroyed by a magnetic field 
of 1 or 2 gauss, directed towards the observer. The magnetic field of the 
earth reduces the percentage of polarisation to less than fifty. Other orienta- 
tions of field produce polarisation of the radiation in directions in which it is 
normally absent, e.g. in the direction of the electric vector of the exciting light. 
Sodium vapour, similarly excited by D,D, radiation, exhibits less than 10 per 
cent. of polarisation in the absence of any magnetic field. This small trace 
of polarisation is destroyed under circumstances similar to those which obtain 
with mereury vapour, except that a field of about 100 gauss is required. With 
the field oriented in other directions the percentage of polarisation is increased 
to thirty or mcre. 
With the exciting light (electric vector perpendicular to the plane of paper) 
and the magnetic field parallel and in the plane of the paper, we have strong 
polarisation in directions perpendicular to the plane of the paper. Jn the 
absence of magnetic field no polarisation would be exhibited in this direction. 
If the field is rotated through 90°, remaining in the plane of the paper, the 
plane of polarisation turns with the field, the electric vector of the resonance 
radiation being horizontal when the field is vertical. If, however, the electric 
vector of the exciting light is in the plane ‘of the paper (i.e. vertical) the 
polarisation diminishes as the magnetic field is rotated, becoming zero with the 
field at 45°, and rising to a maximum again when the field is vertical. These 
relations are difficult to describe without the aid of a diagram. It seems quite 
evident that we are dealing with an orientation of the molecules in the magnetic 
field. 
19. Mr. I. O. Grirriru.—Note on the Measurement of Very High 
Temperature. 
The high temperature is obtained by means of an arc burning in a gas at 
high pressure, and is determined by measuring the ratio of the intensities of 
the light at two wave-lengths. Under a pressure of 80 atmospheres the tempera- 
ture is found to be approximately 8600° absolute. Owing to difficulty in keeping 
the are constant at high pressure this is probably a minimum value, and there 
are indications on some of the plates of the existence of a higher temperature. 
Tuesday, September 18. 
20. Discussion on The Spectra of the Lighter Elements. 
Prorrssor McLennan referred briefly to some theoretical and experimental — 
aspects of the ultra-violet and X-ray spectra of the lighter elements, and 
attention was drawn to fundamental differences in the origin of these spectra. 
The merits of the photo-electric and absorption methods of determining the ~ 
wave-lengths of soft X-rays were discussed, and the advantages possessed by 
the ruled and crystal grating methods were emphasised. 
An analysis was made of the experimental results obtained by the different 
methods with the object of showing that the radiations which atoms of the — 
lighter elements can be made to emit are such as one would expect to obtain — 
with the scheme of electronic orbits provided by Bohr for the neutral atoms — 
of the elements. 
Proressor Bour discussed certain problems connected with the bearing of — 
spectroscopic evidence of an atomic constitution, | 
