AucusT 26, 1915] 

NATURE 
703 

estimate of the density of visible stars in the neigh- 
bourhood of the solar system is 10xsun’s mass in a 
sphere of 5 parsecs radius (525 parsec*), i.e. 0-019 x 
sun’s mass per parsec*. It follows that the density 
of “‘uncondensed”’ or ‘‘residual’”? matter existing in 
interstellar space is of the order 10° that of ‘‘con- 
densed”’ stellar matter. Even if, as there is some 
reason to believe, the number of “dark” stars is very 
much greater than the number of bright ones (Linde- 
mann’s estimate is 4000),*° the ratio referred to is 
still very large. 
It is evident that unless this ‘‘residual’’ or 
“‘primordial’’ gas is exempt from mutual gravita- 
tion** it must give rise to a gravitational field very 
much greater than that of the whole sidereal universe, 
and should therefore be taken into account in existing 
theories of stellar dynamics. Although the dynamics 
of such a system would probably have to be modified 
to a considerable extent to take into account radiation 
pressure, we should still expect an enormously high 
density near its’ mass-centre, unless the whole be 
endowed with a small angular velocity, as is surmised 
to be the case with the Milky Way. 
It follows from this brief discussion that we are 
either obliged to accept the existence of a widespread 
distribution of enormous quantities of interstellar gas 
of molecular density of the order 10* molecules per 
em.? and take into account its influence in stellar 
dynamics, or conclude that the attenuation of light by 
scattering is very much less than is indicated by 
existing estimates of the absorption of stellar radiation 
in space. Louts Vessor KING. 
McGill University, Montreal, June 30, 1915. 

The Great Aurora of June 16, 1915. 
In reply to Dr. Chree’s note in Nature for July 22 
concerning the auroral display of June 16, I would 
say that the times (as indicated in my note in NaTuRE 
for July 15) were in Greenwich Mean Time. This, 
of course, begins at Greenwich Mean Noon. It did 
not occur to me that this might be misleading to the 
unastronomical reader. If one will subtract twelve 
hours from the times given by me, he will then have 
the dates for the morning of June 17 at Greenwich. 
Thus June 16d. 15h. 30m. G.M.T. will be June 17d. 
3h. 3om. a.m., Greenwich Civil Time. 
I will take the opportunity to quote here from the 
Los Angeles (Cal.) Tribune of June 18, 1915, a 
despatch from Chicago dated June 17 :— 
“Chicago telegraph operators were puzzled to-day 
when their wires failed at times to work. Soon, 
however, the explanation of the trouble came. It was 
not due to power stations or lack of current, but to 
the aurora borealis. 
“The bewildering beauty of the northern lights 
lighted up all of Canada and the north-western part 
of the United States last night and caused electrical 
disturbances that put telegraph wires entirely out of 
commission in Idaho, Montana, and the Dakotas and 
along the Canadian Pacific railroad. 
“The disturbances extended to Chicago to-day and 
to-night, and reports of trouble between Pittsburgh 
and New York, St. Louis and Kansas City, and 
many other cities over the country, were received. 
Operators here stated to-night that the disturbance 
was the worst they had known in five years. 
“WW. F. Weber, wire chief for the Western Union, 
reported service considerably demoralised. 
““€ Our wires the whole length of the land were 
badly affected early to-day,’ he said. ‘They are 
affected still, though not to such a degree. The dis- 
turbance of the atmosphere causes fluctuation of the 
20 Lindemann, F. A., “ Note on the Number of Dark Stars,” Monthly 
Notices, \xxv. (1915). 
21 On this point note a remark by Eddington, Joc. cit., p. 258. 
NO. 2391, VOL. 95] 
‘mission of signals. 

current on the wires, and interferes with rapid trans- 
We have been obliged to operate 
at a much lower speed than normal to-day.’ 
“The Postal Telegraph Company was 
affected. 
“Tt also was reported that train-dispatching on the 
Canadian Pacific railroad virtually ceased for several 
hours. Similar conditions prevailed on other northern 
transcontinental lines.” E. E. Barnarp. 
Yerkes Observatory, Williams Bay, Wisconsin, 
August 4. 
similarly 

Use of Celluloid in Periscope Mirrors. 
I sHouLD be glad to know, in reference to the possi- 
bilities of diminishing the danger in using periscopes 
under fire: (1) whether experiments have been tried 
as to the effect of cementing a plate of celluloid to the 
back of the exposed mirror in preventing or reducing 
the splintering of the glass when struck by a bullet, 
and, if so, with what result; (2) whether there is any 
danger involved in the use of celluloid for this purpose ? 
Epwarp M. Lanctey. 
48 Waterloo Road, Bedford, August 17. 

Foreign Philosophers. 
One of the original objects of our Association for 
the Advancement of Science was to encourage the 
exchange of ideas with foreign philosophers, vide 
First Report of B.A. This year it will be a disap- 
pointment to many people in Manchester to have so 
few distinguished strangers. We now call them 
prisoners of war or alien enemies. But they still 
wish for scientific enlightenment. 
Inquiries made in the prisoners’ camp at Stobs 
reveal a small library carefully catalogued. They 
have some hundreds of English books, including some 
scientific books; also, in German, Schiller and Goethe. 
They ask particularly for Naturwissenschaft in 
Deutsch, Chemie, Physik, Geologie, Botanik, also 
agriculture (Landwirthschaft), navigation, engineering, 
mathematics, mathematical astronomy for seamen, 
logarithms, Electrotechnik. There are repeated in- 
quiries for a German book on spherical trigonometry, 
enough copies for the navigation class. 
The requests we have the honour to transmit may 
be satisfied by sending books direct to Von Vorman, 
Librarian, Prisoner of War, Hut 18, Compound A, 
Stobs, near Hawick, Scotland. Inquiries as to books 
likely to be welcome in other camps may be addressed 
to the Emergency Committee, 169 St. Stephen’s 
House, Westminster Bridge, S.W. 
Some of the prisoners have already expressed a 
general willingness to remind their friends in Ger- 
many (with whom they are privileged to communi- 
cate) that the English prisoners in German camps are 
also asking for books. 
Books sent by passenger train should be carriage 
paid, by parcels post they go free of charge. 
August, I9I5. Hucu RicHarDson. 

French Magnanimity. 
FRENCH history furnishes an interesting parallel to 
the magnanimity shown by Napoleon to the Univer- 
sity of Pavia referred to in your issue of July 15. 
When Rudyerd was engaged in building the second 
Eddystone Lighthouse a French privateer captured 
some of his workmen and carried them prisoners to 
France. Louis XIV., however, as soon as he heard 
of it, put the captain and crew in prison, released 
the workmen, loaded them with presents, and sent 
them home, saying that though he was at war with 
England, he was not at war with mankind. 
Gorpon D. Knox. 
11, Garrick Street, W.C., August 17. 
