520 
‘NATURE 
[SEPTEMBER 28, 1899 
Remarkable Lightning Flashes. 
Last September you kindly published a photograph of a 
multiple lightning flash taken with a moving camera. I now 
enclose a photograph taken at Johannesburg by Mr. G. H. 
Preston. Ithink that he must have moved his camera (Frena) 
unintentionally, being startled at the commencement of a very 
vivid flash, which seems to have lasted some considerable time, 
say nearly one second. At any rate there are nine distinct 
lightning flashes, all of identical shape. The first three, or 
perhaps a few more, are very strong and close together, and 
possibly while they were taken the camera may as yet have been 
fairly stationary ; its axis then moved in spiral curves while 
the remaining six flashes imprinted themselves. 
The photograph is of interest on account of the large number 
of individual but otherwise identical flashes, and especially 
because it shows that these individual discharges follow each 
other at irregular intervals ; and I hope that the suggestion which 
I made last year to study this subject may be carried out. 
As on my last year’s photograph, there is on this one an addi- 
tional flash, which appears to be single but is much more 
branched than the other one. C. E. STROMEYER. 
Lancefield, West Didsbury, August 21. 
In NATURE for September 14 (p. 460), the writer on dark 
lightning refers to the absence of dark flashes in pictures of 
artificial discharge. I may perhaps draw the attention of any 
who are unfamiliar with Lord Armstrong’s ‘‘ Electric Move- 
ment in Air and Water”’ to Plate 34 in that book, in which is 
shown a very fine example of a dark flah. Lord Armstrong 
describes how he obtained the discharge on p. 41. Plate 18 is 
also of interest in this connection. HENRY STROUD. 
The Durham College of Science, Newcastle-upon-Tyne, 
September 18. 
I po not see on what grounds it is concluded (p. 423) that ribbon 
lightning has a real existence. The appearance might easily be 
caused by defective vision. If the fork is not distinctly focussed 
on the retina, it may appear either broadened or double or 
multiple, especially if there is any degree of cataract in the eye. 
The ribbon appearance in the photograph shown in your 
article is surely to be explained by the camera having been 
moved downwards and slightly to the right, or else in the op- 
posite direction, and three or more discharges having taken 
place during thetime. The horizon not being sharp is further 
evidence. One may imagine, however, that an appearance of 
this kind might also arise from a discharge being repeated 
through the same air, but the air moving bodily between one 
discharge and the next: it seems to me it yet remains to be 
proved whether such a thing ever does occur. One would sup- 
that if it did, the motion of the air would not be uniform 
throughout the flash, and therefore the ribbon would be unequal 
in width in different parts. 
NO. 1561, VOL. 60] 
pose 
With regard to apparently black lightning, some months ago 
I saw a black fork having exactly the appearance of an ordinary 
flash of forked lightning, only dark on the light ground ofa 
flash of sheet lightning. I concluded the explanation to be that 
given by Lord Kelvin, only the curious circumstance was that I 
did not remember I had previously seen a bright fork of the 
pattern of the dark one. I haveno doubt, however, that there 
must have been one, and that my being dazzled by it caused me 
to see it again, dark, as soon as there was a light background 
to show it. T. W. BACKHOUSE. 
West Hendon House, Sunderland, September 18. 
Ir is surprising that such a brilliant experimentalist as Prof. 
R. W. Wood does not allude to that peculiar reversal of the 
photographic image known as the Clayden effect. 
I drew particular attention to this explanation of the dark 
flash in a lecture before the Royal Photographic Society this 
year, which is fully reported and illustrated in the Photographic 
Journal for March last. 
The Clayden effect is easily verified in the following way. 
Arrange the sparking terminals of a coil, horizontally, about 
four inches apart, with a dark background of velvet ; focus a 
camera for the sparks, then darken the room. Place a strip of 
white card one inch. wide near one terminal in the spark gap, 
uncap the lens, and expose on the card by burning one inch of 
magnesium wire ; then remove the card and pass a spark, now 
place the same card near the other terminal of the spark gap, 
and burn another inch of magnesium wire. On developing 
the plate it will be found that the spark image is reversed over 
the latter card only. 
‘This shows that the same amount of iog has a very different 
effect, whether it is deposited before or after the image. It 
must sometimes happen that in photographing lightning some 
| sky fog or other fog will be deposited after the image ; it 
therefore seems highly probable that any bright flash could be 
converted into a dark flash by slightly fogging the plate before 
development. The Clayden effect also explains why, with a 
number of flashes on the same plate, some may be dark and 
some light, and yet dark lightning probably has no real existence. 
156 Clapham Road, S.W. F. H. GLew. 
Sedge-Warblers seizing Butterflies. 
OBSERVED instances of birds capturing butterflies are so few 
that I venture to think the following worth putting on record. 
On the evening of August 12, at about 6.30 p.m., I was walking 
beside a dyke on Ludham Marsh, Norfolk, when my attention 
was attracted by the alarm notes of a pair of sedge-warblers in 
the reeds. I stood still, and soon caught sight of both birds within 
about six yards of ne. Each had a butterfly in its mouth, and 
with my field-glass I was able to identify the species as a 
meadow brown (Z. _/aniva) and a small white (P. rafae). From 
the behaviour of the birds, and my observation of them on 
subsequent days, I have no doubt that they were feeding their 
nestlings, though I was unable to find the nest. I may add 
that at the time most of the butterflies had taken up their 
quarters for the night on stems of reeds, &c., and that very 
many of the butterflies which I observed during the daytime on 
the marshes had very ragged and chipped wings. These injuries 
may have been caused by wind and contact with twigs, thorns, 
&c., but they were quite compatible with repeated ineffectual 
pecks and snips from the beaks of small birds. 
OswaLp H. LATTER. 
Charterhouse, Godalming, September 17. 
Explosion of Aluminium Iodide. 
T HAD two samples of aluminium iodide in two hermetically 
sealed glass tubes sent by a German firm. One of them was 
passing round the class, and the other was lying on the demon- 
stration table. Suddenly a report was heard, and I found that 
the tube on the table had exploded, and its contents had been 
thrown out. Both the tubes were perfectly sound, and therefore 
there seems to be no reason to suspect that the volatile com- 
pound found an explosive mixture with the air. The temper- 
ature of the lecture room was at the time nearly 95° F. I com- 
municate this matter to you to find out if others have had 
similar experience with aluminium iodide P. L, NARASU. 
Christian College, Madras, July 30. 
