414 
NATURE 
«4 
[Sepé. 3, 1885 
molecules of some phosphorescent substance to be in a state of 
constraint, and to relieve themselves, thus causing vibrations which 
are communicated to the ether—the whole change taking place 
so quickly and on so small a scale that the statistical law above- 
mentioned does not apply, and is not therefore broken. Nay, 
further, I can imagine an enclosure, the walls of which are 
coated internally with an excited phosphorescent body perform- 
ing for all practical purposes the part of an enclosure of low 
temperature under the theory of exchanges, and yet it may be 
continuing for some time to emit visible rays. 
(2) I can, however, imagine the following question to be put : 
Let there be a phosphorescent substance which is capable of 
being excited by certain rays coming from a black body at the 
temperature T, these rays being apparently converted into others 
of lower refrangibility which continue to be given out for some 
time by the phosphorescent body. Let us further suppose that 
the phosphorescent body does not suffer chemical decomposition 
at the temperature 7. 
Now imagine a temperature enclosure kept at temperature T, 
the interior walls of which are lined in part with this phos- 
phorescent substance. What will happen in this enclosure? 
I think there can be little doubt that if there be such an 
enclosure capable of existing permanently and without decom- 
position of the substances which compose it, then the rays which 
it gives out must be those required by the theory of exchanges, 
But if the further question be asked in what way does the 
phosphorescent body conform to the theory of exchanges, we may, 
I think, plead ignorance. As far as Iam aware we have experi- 
mentally little or no knowledge of what the phosphorescent 
substance will do under these conditions, presuming that it can 
exist undecomposed. All our knowledge is limited to its be- 
haviour at a low temperature when acted on by high temperature 
rays, and its peculiar behaviour under these conditions cannot, I 
think, be viewed as a valid objection to the theory of exchanges. 
BALFOUR STEWART 
The Eleven-Year Meridional Oscillation of the Auroral 
Zone 
Tus very remarkable law, in favour of which Mr. Tromholt 
quotes a short series of observations made at Godthaab, which, 
he says, are supported by a few in other Polar regions, would, 
it seems to me, if satisfactorily proved, not only advance the 
science of terrestrial magnetism a stage, but also materially help 
to elucidate the exceedingly mysterious bond of union between 
the aurora and weather. As long as we simply knew that the 
manifestations of the Aurora Polaris increased and diminished 
everywhere with the spotted area of the solar surface, we were 
obliged to conclude that there was a similar increase and 
decrease in the electrical energy of terrestrial currents, and 
meteorological evidence did not favour the idea that the eleven- 
year variation in terrestrial currents was on such an extensive 
scale as the amplitude of the auroral oscillation would imply. 
But now if the law which Tromholt has indicated, really exists, 
a great deal of the difficulty in correlating the two phenomena 
disappears, since it is obvious that a comparatively small dis- 
placement of the zone would cause the annual average number 
of aurore to increase or diminish by their normal amount. 
Thus from lat. 60° N. to lat. 65° N., a distance of only 350 
miles, the annual average number of aurorce diminishes from 
80 to 4o. 
T will not now dwell upon analogous eleyen-year oscillations 
of isobars, such as Blanford’s Asiatic seesaw, and the indications 
of similar secular displacements of the Atlantic isobars noticed by 
Allan Brown and others, or upon the extraordinary resemblance 
in form between the auroral zone and the mean storm track of 
the northern hemisphere charted by Prof. Loomis in his latest 
contribution to meteorology; but I would merely say. that 
Tromholt’s discovery seems likely to become the touchstone 
which may, in the hands of an intelligent and comprehensive 
worker, clear up the entire question, and I earnestly hope that 
no efforts will be spared to corroborate it. 
I will conclude by adding my mite. In looking over Fritz’s 
monograph on the connection between solar spots and terrestrial 
magnetism and meteorology, I have found a series of observa- 
tions at Godthaab and Jakobshayn (69° 22’ N.) further north, 
which do not appear to have been utilised by Mr. Tromholt, 
and which, when combined in the form of percentages, cover a 
space of ten years, and add strong corroboration to the law indi- 
cated by Tromholt.* 
5 5 5 a 2 9 
“Ueber die Beziehungen de: Sonnenfleckenperiode zu den magnetischen 
und meteorologischen Erscheinungen der Erde,” p. 48. 
TABLE I.—JWo. of Aurore seen annually at Godthaab and 
Fakobshavn, compared with Wolf's Sunspot Numbers 
Years .. 1840 qr 42 43 44 45 46 47 48 49 50 
Godthiabw./—» iGo!" jog) 484) 87) 7a 32)" ee 
Jacobshavwn. zo “'rs)' rs’ x8 x2 24 ‘ar! x7 x4 7 ix) eee 
Sunspots ... 63°2 36°8 24°2 10°7 1570 ior 61°5 984 124°5 95°9 66'5 
TABLE II.—The above numbers of Aurore converted into per- 
centages of their means and compared after smoothing with 
smoothed Sunspot Numbers * 
Years ... 1840 41 2 3) sak 45 46 47. 48 49 50 
Godthaab... — 84 131 8 SSS 
43 
It 
Jacobshavn. 62 93 93 Tir 74 348 axg0 O05) 86 68 130 
129 
Smoothed 
means of } 70°5 87 106'2 Iog 109'2 1012 95°7 86'2 88 109 
both ] 
Smoothed 4 " . a 5 nD 2 5 5 A « 
sunspots | 544 40°2 23°9 I5'r 202 39°r 65°3. 95°6 110°7 95'6 76'3 
The figures in Table IT. speak for themselves. 
To corroborate this law by further observation will necessitate 
a prolonged sojourn in some region xorvth of the maximum 
auroral zone, and Greenland appears to be almost the only 
region where this could be done in the absence of a regular 
Polar expedition. E. DouGias ARCHIBALD 
Tunbridge Wells 
On Cases of the Production of ‘‘Ohm’s (or Langberg’s) 
Ellipses” by Biaxial Crystals 
IN examining the macled crystals of potassium chlorate, which 
are so extremely common in the ordinary crystallised salt, I 
have found that all those which consist of two hemitrope plates 
only, nearly equal in thickness, give the above-mentioned 
secondary interference-curves when placed in homogeneous 
convergent plane-polarised light. 
This result is no more than we should expect if the crystals 
were uniaxial, as Prof. Largberg showed (ogg. Annalen 
Erginzungsbhd., 1., 540) many years before the curves were 
independently discovered by Prof. G. S. Ohm (see NATURE 
for November 27, 1884, p. 83). But potassium chlorate is a 
biaxial crystal, the angle included by the optic axes being 28° 30° 
(determined in olive oil), and I donot find that the production of 
the curves in such crystals has been hitherto noticed. 
The plane of the optic axes, however, makes so large an angle, 
viz. 38° 30’ (as determined in olive oil), with the normal to the 
surfaces of the plates in which potassium chlorate usually crystall- 
ises, that the isochromatic curves in the vicinity of this normal 
belong to a very high order, and do not sensibly differ from 
portions of circles of large radius. Thus in a macle, in which 
the crystallographic position of one of the components differs by 
180° from that of the other, the planes of the optic axes make 
equal angles of 38° 30’ with the normal on opposite sides of it, 
and so the conditions determined by Langberg for the production 
of the secondary ellipses are fulfilled. I have, in fact, made 
artificial twins of this kind by cementing together plates of the 
salt oriented as above indicated ; and I find that they show the 
ellipses precisely as the natural macles do. Of course, in order 
to see them, the compound plate must be so placed that the 
plane which includes the normal and the two acute bisectrices 
makes an angle of 45° with the plane of polarisation of the light. 
In a good micropolariscope the four optic axes and portions of 
the lemniscates immediately surrounding them are visible at the 
edges of the field. ; 
It is possible, but not common, to find crystals of potassium 
chlorate consisting of three plates nearly equal in thickness, 
the top and bottom plates being symmetrically disposed, 
while the intermediate one differs from them in crystallo- 
graphic position by 180°. In such cases the secondary in- 
terference-curves are much more complicated, two sets of 
ellipses being generally visible, one on each side of the 
centre of the field (the exact ‘position, of course, depending on 
the relative thickness of the plates, as Langberg has shown). 
One macle I have found to consist of five or six distinct plates, 
and the secondary curves produced by this are too complicated 
to be easily described. 
I do not find any marked difference between the curves pro- 
duced by the iridescent twins and those given by the ordinary 
macles. Many of the iridescent crystals show,'when the plane of 
ate2b+e 
<The figures are smoothed by the formula , where 2 is the 
figure for the epoch and @ and c the preceding and succeeding figures. 
