216 
NATURE 
OUR ASTRONOMICAL COLUMN. 
Tue Ecriesinc Binary RX Hercuris.—An extended 
series of observations of photographic magnitudes of 
this variable has lately been made and discussed by 
R. H. Baker and Edith E. Cummings (Laws Obs. 
Bull., No. 25). The observations were made by the 
extra-focal method and, in combination with the 
spectroscopic evidence, lead to the following conclu- 
sions :—The two stars are slightly ellipsoidal, and each 
has a radius about one and a half times that of the 
sun. The mass of each star is a little less than that 
of the sun, and the density is about one-fourth of the | 
sun’s density, The star eclipsed at principal conjunc- 
tion is of magnitude 7-96, and is brighter by o-12 
magnitude than its companion; its surface brightness, 
mass, and density are greater than those of the fainter 
star by 12, 6, and g per cent. respectively, and it is 
of slightly earlier spectral class (A). The distance 
between the centres of the two stars is about five times 
the radius of either star, or about 5,280,000 km. At 
conjunction 7o per cent. of the disc of one star is 
eclipsed by the other. The period is 1-7785740 days 
(Shapley), and the semi-duration of eclipse 2h, 53m. 
The photographic magnitude of the system is 7-264, 
and the magnitudes at primary and secondary minima 
7-84 and 7-71 respectively. Outside eclipse the intensi- 
ties generally increase towards secondary minimum, 
showing that the light of the star eclipsed at this time 
is augmented by radiation of its brighter companion. 
SpEcrroscopic. RESOLVING Power.—The resolving 
power of a spectroscope is limited by the diffractional 
broadening of the geometrical images of the slit, and, 
following Lord Rayleigh, the limit of resolution 
usually adopted is determined by the condition that the 
maximum of the central band corresponding to one of 
the lines should fall on the minimum of that of the 
second line. Under these conditions, for two lines of 
equal intensity, the intensity at the centre of the 
combined bands is o-81 of that of the maxima. An 
experimental investigation made by C. M. Sparrow 
at the University of Virginia (Astrophysical Journal, 
vol. xliv., p. 76) has led to the result that the limit 
of resolution is given by the “undulation condition ’’— 
that is, by the condition that the central minimum shall 
just disappear. The theoretical resolving power -thus 
derived is about 26 per cent. greater than that given by 
the Rayleigh criterion. Among other matters of in- 
terest, a simple approximate formula is given for the 
resolving power of the Fabry and Perot interfero- 
meter, ; 
Tue Minimum RapiaTion Visually PERCEPTIBLE.— 
Adopting the light from a 6th magnitude star as the 
smallest amount perceptible, Dr. H. E. Ives has 
made an interesting calculation of the corresponding 
least quantity of radiant energy capable of exciting 
the sensation of light (Astrophysical Journal, vol. xliv., 
p. 124). Taking Russell’s estimate that a candle at 
one metre distance is of stellar magnitude — 14-18, it is 
easily deduced that the brightness of a 6th magnitude 
star is o-849x10-° of this. Since a metre-candle is 
equivalent to 1-59 ergs per sec. per sq. cm., it follows 
that the least power corresponding to illumination from 
a light-source of the above brightness is 1-35 x 10-* ergs 
per sec. per sq. cm. Assuming 6 mm. as the diameter 
of the pupil, the radiation entering the eye from a 
light-source of maximum efficiency of the brightness 
of a 6th magnitude star would be 0-38 x 10° ergs per 
sec. On the assumptions made, this is the smallest 
amount of radiation perceivable by the eye, but it is 
important to note that the figures given only apply to 
radiation from a distant point-source, such as a star. 
At this rate of energy-reception the eye would receive 
through the pupil the elementary energy-quantum in 
one-thousandth of a second. 
NO. 2455, VOL. 98] 
© "Se 
q 
[NovEMBER 16, 1916 
METEOROLOGY IN WAR.  ~ * 
LONG before the outbreak of the world conflagra- 
tion of the past two years war and the weather 
| Was a subject which captivated many minds, mainly 
| of non-scientific or semi-scientific people who were pre- 
pared to accept as proof the most superficial circum- 
belief. Even during the progress of the present war 
there have been many discussions in the Press and 
| before societies in this and in other countries which have 
had for their object the perpetuation of the old belief 
that gun-firing causes rain, though round Shoebury- 
/ness, our great gun-firing station, less rain falls than — 
,in any other part of the British Isles! These quasi- 
| learned discussions have been of no assistance to the 
fighting forces on sea or land. : 
Scientific investigators, however, have not been idle, 
though little or nothing of their activities is known 
outside official circles. The meteorological establish- 
ments of the various countries involved in the great 
war have been doing their utmost to utilise the now 
very greatly increased knowledge of the physics of the 
atmosphere for the immediate benefit of the armies. 
meteorologists can go no further than prepare, on the 
lines familiar to us during the past half-century, pre- 
| dictions of the weather changes likely to take place 
within the coming twenty-four hours. But the matter 
has got far beyond this. 
annual report of the Meteorological Committee for the 
year ended March 31 last, just issued (Cd. 8381, price 
1d.) :—‘tThe variety of ways in which the weather 
affects warlike operations in all parts of the world 
has become very apparent from the diversity of the 
information which the Office is called upon to supply at 
short notice. The results of meteorological inquiries 
initiated in what appeared to be the remote interest of 
the theory of the circulation of the atmosphere have 
turned out to have important practical bearings.” 
Both for naval and military operations accurate 
ably they are not more important than other tasks 
now undertaken by the meteorologists—for example, 
the behaviour of the upper atmosphere for the in- 
formation of the flying services, and the condition of 
the surface atmosphere and its relation to gun-sighting 
and range-finding. According to the official report 
referred to above a separate unit of the Royal Engineers 
was created for a meteorological field service, that 
with the Expeditionary Force in France being under 
the command of Major Gold, one of the Meteorological 
Office superintendents, and that in the eastern Mediter- 
| ranean under Capt. Wedderburn, honorary secretary of 
the Scottish Meteorological Society, assisted by Lieut. 
Kidson, of Canterbury College, New Zealand, and 
magnetician in the service of the Carnegie Institution 
of Washington. In view of the importance of an 
adequate knowledge of the weather to the proper con- 
duct of naval and military operations in the Mediter- 
ranean generally, Major Lyons, R.E., formerly Direc- 
| tor-General of the Egyptian Survey Department, was 
appointed to take charge of this section. Further, the 
importance of co-ordinating the experience of flying 
officers with the-work of the Office and observatories in 
order to obtain more effective knowledge of the struc- 
ture of the atmosphere has led to the appointment of 
Major G. I. Taylor, Schuster reader in meteorology, to 
the professorship of meteorology to the Royal Flying 
Corps. 
That the responsible authorities appreciate the work 
performed by the meteorological services is evidenced 
by the Distinguished Service Order conferred on Major 
Gold, and the inclusion of some of his assistants in | 
the Commander-in-Chief’s despatches; while the 
z 
2 
stances which seemed to substantiate any popular ~ 
Perhaps, at first sight, it would be natural to infer that 
As is stated in the eleventh 
forecasts have been greatly enhanced in value, yet prob-. 
= 
; 
