418 TRANSACLIONS OF SECTION A. 
6. One frequently sees claims put forward for periodicities of various kinds in 
meteorology and cognate sciences, just as one sees claims for proportionality ; but 
just as in the later case it is now obligatory to give some numerical measure of the 
precision of the proportionality, either by the error of mean square, or by the co- 
efficient of correlation, so it is not too much to say that it is obligatory on claimants 
for periodicity to give a measure of the precision of their claim, Otherwise, by Lord 
Kelvin’s dictum, their claim cannot rank as work of scientific value. 
DEPARTMENT OF GENERAL Puysics. 
The following Papers were read :— 
1. The Pressure Effect as a Means for the Resolution of a Spectrum 
into Series of Lines. By W. Guorrrey Durrieup, D.Sc. 
Photographs were offered showing the different behaviour of spectral lines 
under pressure, which facilitates their grouping. Not only are the ordinary series 
lines affected in this way, but also non-series lines. Grouping is rendered 
possible by the following features which are emphasised under pressure :— 
(1) The general appearance of the line—sharp or nebulous. 
(2) Its relative change of intensity under pressure. 
(3) The nature of its broadening, which may be symmetrical or asymmetri- 
cal, with different degrees of asymmetry. 
(4) The nature of its reversal, which may be symmetrical or asymmetrical 
with regard to the line upon which it is superposed. 
(5) The direction and amount of the displacement. 
It is sometimes possible to subdivide a general group into triplets by means of 
the appearance of the spectrum under pressure. No. 5 affords a means for obtain- 
ing the numerical relations between the different groups. 
There is some evidence for believing that the vibrating systems which produce 
the different groups are different, the various systems showing different variabili- 
ties in the same environment. 
2. Photographs of the Arc Spectrum of Nickel under Pressure. 
By W. Grorrrey Durrietp, D.Sc. 
Photographs of the arc spectrum of nickel have been taken when the pressure 
of the air surrounding the poles of the metal was varied from 1 to 10 atmospheres 
throughout a large range of the spectrum. For a portion of the spectrum 201 
atmospheres have been reached. 
The photographs illustrate the broadening, displacement, and reversal of the 
lines, and afford some means of resolving the spectrum into different groups of 
lines. 
In general the effects resemble those obtained with the iron are under pressure. 
3. On the Calculation of the Vields of Telescopic Object Glasses. 
By Professor R. A. Sampson, F.R.S. 
4. The Optical Rotatory Power of Quartz. 
By T. Martin Lowry, D.Sc., F.C.S. 
The chief difficulties in measuring the optical rotation of light in quartz 
are (1) to secure crystals of quartz which are entirely free from optical faults, 
and (2) to produce light of sufficient purity to give a clean extinction when 
reading a rotation of several thousand degrees and of sufficient intensity to be 
read with a small half-shadow angle. The best tests of the optical purity of 
quartz are (1) to illuminate the quartz plate or rod with monochromatic green 
