Aucust 31, 1899] 
NATURE 
i) 
=) 
RIBBON AND DARK LIGHTNING. 
M®: ALEX. MORTON, secretary and librarian ef 
the Royal Society of Tasmania, has sent some 
photographs of lightning flashes taken by Mr. W. Aiken- 
head, one of which is here reproduced. The photographs 
were taken at night with a hand camera. Referring to 
them, Mr. Aikenhead remarks :—‘“ The thunderstorm 
was an unusually severe one, and the atmosphere sur- 
charged with electricity. as evidenced by the frequency 
and extraordinary vividness of the lightning flashes, 
whose brilliancy momentarily rendered objects, even at 
a distance, as clearly discernible as in daylight. The 
intensity of the ‘triple’ flash—of which I was so for- 
tunate as to secure a counterfeit—was so great that for 
some moments I was completely dazzled. I may mention 
that the thunderstorm lasted fully an hour, and was at 
its height about g o'clock ; and it was at this period the 
exposures were made with my camera.” 
The accompanying picture is interesting on account of 
the triple flash represented in it, and the dark lines 
apparently radiating from it. In an article printed in 
NATURE several years ago (vol. xlil. p. 151, 1890), Mr. 
Shelford Bidwell described each of these characteristics 
of photographs of lightning flashes, and gave explan- 
ations of them. He remarked that in nearly, if not 
quite, every case where broad ribbon lightning has been 
photographed, the camera was held in the operator’s 
hand—a fact which naturally suggests the idea that the 
widened image of the flash may be due to the movement 
of the camera during exposure. Though it might be 
impossible to move the camera appreciably in the brief 
duration of a single lightning flash, several flashes some- 
times pass in quick succession over the same path, so 
that they may appear side by side upon the photograph if 
the camera is shifted during their occurrence. Moreover, 
Mr. Bidwell pointed out that lightning sometimes leaves 
a kind of phosphorescence along its track, and this may 
fast long enough to produce a photographic picture, even 
though the flash itself was instantaneous. A photograph 
of a triple lightning flash reproduced in NATURE of 
October 13, 1898 (vol. lvili. p. 570) furnishes decisive 
evidence that a camera can be moved quickly enough to 
obtain several pictures of a single luminous track of 
lightning. The three flashes shown in that picture are 
identical in shape, and it is estimated that they followed 
one another along the track with a frequency of about 
30-35 per second. 
But while it is certain that some photographs of 
multiple and ribbon lightning are produced by move- 
ment of the camera, others represent actual lightning of 
a broadened or multiple form. Commenting upon some 
photographs of ribbon lightning obtained by the Rev. J. 
Stewart-Smith, Prof. Cleveland Abbe remarked in the 
U.S. Monthly Weather Review of August 1898 that he 
thought that they were not taken by moving the camera 
during exposure. He considered that a discharge of 
lightning was too fleeting to be influenced by the motion 
of the camera. With artificial oscillatory discharges the 
time of the discharges and the motion of the sensitive 
film might be so controlled as to produce the appearance 
of a ribbon ; but no motion of the camera seemed likely 
to explain the many details in the ribbon photographs of 
natural lightning described. Prof. Abbe thought, however, 
there was one flash on Mr. Stewart-Smith’s plate that had 
every indication of being certainly an oscillatory dis- 
charge, showing lines of flow identical with those photo- 
graphed by Prof. Trowbridge at Cambridge, Massa- 
chusetts, and fully maintaining his conclusion that the 
lightning flash is an oscillatory discharge repeated 
frequently to and fro within the crack in the air that is 
opened by the first discharge. 
That lightning flashes can actually present a ribbon- 
like appearance, and have an appreciable duration, is 
NO. 1557, VOL. 60] 
borne out bya letter which was sent to the Royal Society 
from Buluwayo at the end of 1895, and was printed in 
NATURE of January 23, 1896 (vol. liii., p.272). The 
writers state that they were sitting in a room when one 
of them called attention to a very bright lightning flash. 
“All of us promptly went to the door, whence we wit- 
nessed a truly extraordinary sight in the shape of three 
ribbons of a greenish-white lightning, which hung in the 
sky, motionless, for what must have been fifteen to 
twenty seconds. It seemed to be a long way off (in a 
north-westerly direction), as we heard no report of 
thunder whatever. There could be no mistake about it 
—it was as distinct as possible, and it must have lasted 
fifteen seconds a¢ /eas¢.” With evidence of this kind to 
consider, the reality of the ribbon appearance cannot be 
doubted. To obtain more definite information concern- 
ing this form of lightning and the nature of the electric 
discharge in an ordinary lightning flash, systematic at- 
tempts should be made to photograph lightning with 
cameras having a known rate of movement, and an 
Photograph of lightning taken at Devonport, Tasmania, by Mr. Aikenhead. 
arrangement for determining the angular diameter of the 
ribbon. 
As to the dark ramified flashes shown upon the ac- 
companying picture, Mr. A. W. Clayden has shown by 
experiment that they are due to photographic reversal. 
If the lens of a camera is covered up immediately a flash 
has been photographed, the flash comes out bright in the 
ordinary way in the print ; if, however, the lens is allowed 
to remain uncovered for a minute or so, thus exposing 
the plate to the diffused light of the sky or the glare of 
other flashes, the original flash appears black upon the 
final print. 
In the same way, the discharge of an electrical machine 
can be made to appear dark in a photograph by leaving 
the lens uncovered for about a minute after the discharge 
has imprinted itself upon the plate. According to this, 
the dark ramifications in Mr. Aikenhead’s picture repre- 
sent a discharge of lightning which occurred before the 
bright triple flash. The glare of the bright flash and the 
