JuNE 8, 1899] 
NATURE 
131 
be that we are displaying a crass ignorance in endeavour- 
ing to apply to auroree methods of measurement which | 
depend for their success upon an apparent displacement, 
due to a real change in the position of the observer. 
Students of trigonometry are taught at a very early 
stage the method of determining the distance of an 
inaccessible object, visible from two positions, and the 
elementary process employed, depending on the solution 
of a triangle, remains the favourite method of deter- 
mining the height of the aurora. But just as our 
student knows that a successful solution of the problem 
demands that the angles must refer to a concrete 
object, so the observer of an aurora asks that this 
fitful light should have a definite “locus,” that can be 
simultaneously seen and identified by two or more 
observers. 
not always easily satisfied, and other methods have in 
consequence been suggested which are founded on a 
supposed knowledge of the origin and behaviour of 
the auroral light. We may say at once that these 
methods, often ingenious in themselves, are so unsatis- 
factory in application that they can be passed over with 
a very brief mention. 
There is no doubt but that the light out of which the 
auroral phenomena are formed emanates from a certain 
circumscribed region, but the real question is whether 
the arches and beams, the streamers and waves, the 
curtains and folds, with all the varied nomenclature that 
has been used to describe special features, are definite 
concrete objects. The fact that the aurora is accom- 
panied with a special and presumably constant spectrum, 
possessing easily recognisable characteristics, does not 
help us at all to settle the question. That we have a 
source of light is admitted. The point at issue is, to 
what extent is it a subjective phenomenon, and how far 
does each observer see his own aurora as an optical 
illusion. Manifestly, those who accept the subjective 
theory have to encounter many objections. No one likes 
to admit that he is deceived in the character of a 
phenomenon so apparently real as that presented by a 
fine auroral display, though he will readily acknowledge 
that perspective must introduce some misleading features. 
Prof. Cleveland Abbe has, however, summed up the 
evidence with great care and completeness, and come 
to the conclusion that the idea of an individual existence 
must be definitely relinquished. He has been led to this 
conclusion from an examination of the various attempts 
that have been made to determine the height of the | 
aurora ; and whether we accept this decision or not, we 
shall at least be prepared to follow him in the assertion 
that the determination of the altitude of the aurora isa 
much more delicate problem, and perhaps also a more 
indefinite problem, than we have hitherto believed. 
The evidence tending to this latter conclusion can be 
divided under many heads. We shall content ourselves 
with exhibiting two—one depending upon actual ob- 
servation and measurement, the other resting upon 
theory and suggestion. The observers who have made 
the height of the aurora a special study can be grouped 
into two families—one represented by Richardson, 
Franklin, Hooker, and Silberman, who have actually 
seen the aurora below the clouds, or between themselves | 
and neighbouring objects; and others like Loomis, 
Boscovich, and Twining, who place the height anywhere 
between 400 and 1000 miles. Between these advocates 
for a “ground” theory, and those who perceive a high 
aerial origin, we have a whole host of observers who are 
mainly led to their results by the selection and rejection 
of certain of their observations, if they are numerous, 
or have drawn their conclusions from single and acci- 
dental results. For the statement of claim of those who 
argue that the aurora is entirely confined to the lowest | 
stratum of the earth’s atmosphere, we must trust entirely 
NO. 1545, VOL. 60] 
Practically, this fundamental condition is | 
| 
to description. Measurement can evidently play no 
part, any more than it can on a bank of fog or a shower 
of hail. An admirable description, and one that would 
carry conviction to every impartial reader, if we could 
give it fully, has been written by Prof. J. P. Lesley, the 
distinguished geologist, of what he saw at Little Glace 
Bay, about seventeen miles from Sydney, Cape Breton : 
“It was my good fortune to observe an aurora, which to 
my eyes was embodied in and swept the earth with suc- 
cessive banks of Cape Breton fog. . . . In this fog bank 
hung, as it were, a brilliant curtain of light, with a wide 
fringe or flounce of maximum brilliancy, along the 
bottom edge, the light fading upwards along the curtain, 
but traceable to the very zenith, and the curtain stretch- 
ing from the eastern horizon out at sea to the western 
horizon on the low hill-tops. The perspective was per- 
fect. The curtain was evidently vertical, thin, straight, 
long enough to reach from one limit of the vision to the 
other, and floating broadside before the south wind 
towards the north. No reasoning could convince us 
| (he had a companion) that these were not elements of 
the phenomenon, and, moreover, that the lower edge of 
the bright fringe was more than one or two hundred 
yards away at its nearest point when we first saw it. Its 
rate of departure from us was evidently that of the fog 
bank, or that of the gentle south wind then blowing. 
The perspective of the whole curtain changed in con- 
formity with that supposition. We had both spent our 
lives in topographical work, and no record of tri- 
angulation made upon this aurora would alter my con- 
viction of the posture and movements of the beautiful 
object, derived from the natural triangulations of the 
unassisted eye.” Prof. Lesley further relates that he 
witnessed successive repetitions of the same beautiful 
appearance, but feebler in intensity, as though produced 
by the same causes gradually growing less and less 
active, from a process of exhaustion. Of the accuracy 
of his testimony he can entertain no doubt, and urges 
that it is unreasonable that the positive observations of 
those who have witnessed these displays should be de- 
spotically overriden by the trigonometrical calculations 
of other students. We are inclined to agree with him. 
It might, of course, be urged by those who consider that 
the upper and attenuated regions of the atmosphere are 
necessary for the production of auroral light, that such 
an exhibition was not a true aurora, and that if examined 
spectroscopically the light would not show the character- 
istic lines, nor would the magnetic instruments in the 
neighbourhood be agitated in the manner with which 
| we have been made familiar when aurore are present. 
On these points there seems to be no evidence, nor, so 
far as is known, have other physicists, who, like General 
Sabine, have “ walked through an aurora as one would 
pass through a mist,” verified their convictions. j 
But if the deductions of those who trust to the evi- 
dence of their senses can be set aside as affected by 
self-deception, others who rely on elaborate measure- 
ments are hardly in better case. With more pretentious 
methods, more rigorous criticism can be applied. Into 
| the details of this criticism it is not convenient to enter 
here, involving as it does that much-debated quantity 
the “probable error,” and still more recondite criteria 
for the rejection of discordant observations. But we have 
a right to expect an intelligible result, and this is not in 
every case forthcoming. If a man carefully surveyed a 
field with the view of determining its superficial area, 
brought out as his result a minus quantity, we should 
necessarily have a difficulty in explaining his deductions. 
And, without any exaggeration, it is precisely results of 
this character which are too frequently obtained from 
attempted measurements of auroree. Of course, in the 
case of an object so ill-defined as an auroral arch, one 
expects to find large observational errors. But these 
