OcTOBER 12, 1899} 
IVA TURE 
5a3 
however, is the veversa/, which extends nearly the whole 
way up the centre—that is, the dark flash has along its 
centre a bright core. It is this very photograph which 
made me cast doubt on the hypothesis suggested by 
Mr. Clayden, for both a very strong flash can be recorded 
very dark (with a reversal), and also a weak flash (see 
Fig. 2, B). If the reader refers to an interesting article 
on “Lightning,” by Mr. Jeremy Broome, that appeared 
in the January number of the Strand Magazine in 1897, 
there will be found a reproduction of a photograph taken 
at Cambridge by Messrs. Valentine Blanchard and Lunn, 
showing a éright flash with a dark reversal down the 
centre, the exact opposite to the flash recorded above. 
It may be remarked that a reversal is perfectly distinct 
from a double flash, many of which have been recorded. 
Another flash of interest and peculiarity is that marked | 
c. This flash is quite distinct from B, but unlike all the 
other bright flashes of about the same intensity, which 
are clear and sharply defined, this one is occasionally 
split up along its path into two parts, and the flash on 
both sides throughout its whole length is bounded with 
dark borders. Both the original negative and a silver 
print show this peculiarity distinctly, but unfortunately | 
the dark borders are lost in the reproduction. I find 
that this peculiarity about a flash has been photographed 
before, but apparently not noticed. If the reader will 
refer to an old number of Avow/edge (vol. xvill. p. 224), 
he will find a reproduction of a lightning flash taken by 
Mr. George Primavesi at Tooting. This flash is far 
more intense than that on my negative, and the dark 
borders are more developed. The main stream is 
devoid of ramifications: the exposure lasted for only 
one second. 
To sum up, then, the different appearances of the | 
lightning flashes recorded in these photographs, and 
others of which I possess either photographs or re- 
productions ; we have the following various kinds :— 
Main stream. eeminca | Peyersal | Source of information. 
Bright None | Lie LGN 
- | Bright Fig. 4, B 
e Dark } NATURE, vol. Ix. 
- 423 
ac _ None Dark Strand Magazine, 
Tan. 1897, p. 41 
7 Bright Dark ? 
Aa | Dark Dark ? 
Dark None Fig. 3, A 
a beDark , . | Fig: 2, a 
| Bright ? 
a5 None Bright 2 
oe Dark | Bright Fig. 4,.A 
» Bright | Bright ? 
under which category it should be placed. 
Now in attempting to explain the cause of dark light- 
ning I employed Mr. Clayden’s idea as a_ working 
hypothesis, but I can find no reference to any illustra- 
tions of the experiments he carried out. Mr. Shelford 
Bidwell, however (NATURE, vol. xlii. p. 153), describes 
and illustrates one out of a series of €xperiments he made, 
and this shows dark and bright flashes made artificially, 
but the flashes are simply dark or bright, with no other 
peculiarities. 
Further, in a letter which appeared ina very recent 
issue of this journal, Mr. F. H. Glew mentions that 
he also has made several experiments with re- 
gard to the Clayden effect. The illustration which 
NO. 1563, VOL. 60] 
accompanies his account of these investigations (the 
Photographic Journal, vol. xxiii. No.7, p. 179) shows, like 
Mr. Bidwell’s, no more than simple dark and bright 
flashes. I may here mention that the method described 
by me further on was not very dissimilar to that employed 
by Mr. Glew, although I was unaware until quite recently 
of the publication of his to which reference has just 
been made. 
Now the point most interesting to me was, Could one 
artificially produce on one plate or film exact types 
of dark and bright flashes as shown in the above 
illustrations ; that is, flashes which are dark with d7zght 
cores and bright with dark borders? No photographs of 
sparks produced artificially have, so far as I know, 
displayed any of these peculiarities. 
I will simply describe one experiment that I made, with 
this object in view, in the laboratory of the Solar Physics 
Observatory, Kensington. 
To produce the spark I employed a 10-inch Apps’ coil, 
with a pint jar in circuit, fed by two cells of four volts 
each, the sparking distance being two inches. The 
camera was a small 5 x 4 by Herr Winkel of Gottingen, 
fitted with a Zeiss objective. Although it was made only 
fic. 6.—Showing three series of sparks taker on one plate against a white 
background. During the passage of the sparks at c, the background 
was artificially illuminated, 
for the use of glass plates, by a simple device Eastman’s 
film could be employed. Films, I may mention, eliminate 
all chances of halation. 
The method of procedure was as follows :— 
In a darkened room I first of all made an exposure on 
a single (2-inch) spark against a bright (white card- 
board) background. On development this bright flash 
came out naturally drzght. : : 
I next inserted a new film and repeated the same 
experiment; except that I did not remove the film or 
develop it immediately. Covering up the lens carefully, 
I moved the poles in a vertical plane so that the next: 
spark should fall on a different part of the film, and 
made a second exposure on two sparks. Again covering 
the lens, and moving the poles a little in the same » 
direction, I exposed the film once more to a series of 
four sparks, but while I allowed the sparks to pass I 
illuminated the cardboard background by burning one 
inch of magnesium ribbon at a distance of two feet. 
It may be mentioned that the poles only appear on the 
negative in their respective positions when the back- 
ground is artificially illuminated. Fig 6 shows the 
results obtained. A is the first spark, B the two sparks 
after the first movement of the poles, and C the last four 
flashes when the background was artificially illuminated. 
