460 
NATURE 
[SEPTEMBER 14, 1899 
growth stages were in 1884 described and discussed by 
Sladen, and conjures up theories of stalked-ancestry, 
pentameral symmetry, and the like. Whatever the 
significance of the facts, this beautiful report comes to 
us at an opportune moment, @ propos of the issue of the 
magnificent series of “Illustrations,” delineating the 
many novel forms discovered and described by Dr. 
Alcock and his co-workers in the Indian seas during 
the last nine to ten years. He has shown by his own 
share of the work that it is possible for one man, new to 
the task of marine investigation, to successfully handle 
taxonomically groups so dissimilarly ordained as the 
Bony Fishes and Echinoderms, to say nothing of his 
sterling work upon the “Carcinological Fauna” of the 
area. An achievement this of which he may well be proud. 
The Annals and Magazine of Natural History and 
Journal of the Asiatic Society of Bengal for the period 
named teem with his original communications, and to 
him, to Commanders Carpenter, Hoskyn, and Oldham, 
to Dr. A. S. Anderson, who has more recently taken up 
the work, their collaborators, assistants, and native 
artists, we tender our hearty congratulations upon the 
skill and persistent patient enthusiasm with which they 
have so long and so successfully continued their task. 
Work thus performed is always durable, and that of 
H.M. Survey ship Jzvestigator will be ever con- 
spicuous among post-Challengerian explorations of the 
deep sea. 
We close the report with a feeling of gratitude to all 
concerned in its production. 
LETTERS TO THE EDITOR. 
(Zhe Edvtor does not hold himself responsible for opinions ex- 
pressed by hts correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts tntended for this or any other part of NATURE. 
No notice is taken of anonymous communications. | 
Dark Lightning. 
I HAVE been greatly interested by some photographs showing 
the rare phenomenon of dark lightning, which have recently 
been sent to me. So far as I know the only explanation that has 
ever been offered to account for them is photographic reversal, 
due to extreme brilliancy. This appears to me to be wholly 
out of the question for two reasons. In the first place, a dark 
line on the picture, resulting from Over-exposure of a very 
brilliant line, would be surrounded by bright edges due to the 
lesser photographic action in the halation region. This is never 
present so far as I know, the dark flashes being minute black 
lines ramifying from, or in the neighbourhood of, the main 
discharge. Secondly, from what evidence I can gather, the 
dark parts of the flash are not those which appear most brilliant 
to the observer. Mr. Jennings, of Philadelphia, who in 1890 
secured a remarkable picture (reproduced in Photog. Times 
Annual, 1891) showing a very brilliant flash with countless 
dark flashes covering the sky around it, tells me that the 
appearance to the eye was a brilliant white discharge, with 
fainter rose-coloured ramifications, the latter developing in the 
negative, or rather positive, as dark flashes. 
Some years ago it occurred to me that a dark flash might be 
produced by a3preponderance of infra-red radiations, which, as 
Abney has shown, undo the work of ordinary light on the plate. 
If we had a form of discharge capable of giving off very little 
actinic light, and an abundance of infra-red light, it might come 
out dark on a feebly illuminated background. This is of course 
a very wild guess, with nothing to substantiate it ; but the dark 
flash appears to be a reality, and a poor hypothesis is 
perhaps better than none at all. I have recently thought 
that the phenomenon might perhaps be explained in another 
way. 
We have a flash which appears darker than the sky behind it. 
It is inconceivable that the discharge could render the air in its 
path opaque, in the ordinary sense, to white light. But the 
NO. 1559, VOL. 60] 
light which illuminates the sky, in the case®of these pictures, is 
not daylight, but light coming from another flash, that is made 
up of wave-lengths corresponding to the periods of vibration of 
the dissociated matter in the path of the discharge. Now, 
may it not be possible that in the dark flash we have a 
discharge, weak or nearly wanting in actinic light, which, 
however, renders the air in its path capable of absorbing to 
some extent the radiations of the wave-lengths which come 
from the bright flash ? 
Such a flash might possibly appear dark on a background 
feebly illuminated by light, exclusively of these wave-lengths. 
In other words, may wenot have in the path of the dark flash 
dissociated molecules, radiating but feebly, and capable of 
taking up vibrations of periods similar to their own, coming 
originally from a simultaneous brighter discharge ? 
It might not be impossible to reproduce the phenomenon by 
photographing a spark in front of a light background. Sparks 
are almost always taken against a black background, which 
would account for the absence of dark flashes in pictures of arti- 
ficial discharges. A heavy main spark with lateral branches 
would seem the most suitable kind to employ. 
The best method of attacking the problem experimentally, it 
seems to me, would be a search for selective absorption in a 
partially exhausted tube. 
If the source of light were continuous, any absorption would 
be unnoticeable, unless persisting for some time after the dis- 
charge (which is unlikely), for the time between successive 
discharges is great in comparison to the actual duration of one of 
them. Even in the case of so-called continuous discharges 
produced by high potential batteries, the discharge is 
often, and may always be, intermittent in character. The 
source of light should then be of no longer duration than 
the discharge occurring in the gas the absorption of which 
is to be examined. 
Ican think of no way of producing a white or continuous 
spectrum source of as snort duration as, and contemporaneous 
with, the discharge in the tube, but by employing two tubes dif- 
ferently excited, the one as a light source, the other as an ab- 
sorption tube, some results might be obtained. 
Prof. Trowbridge found that an argon tube emitted a blue 
light or red light according to whether it was illuminated by 
means of an oscillatory or non-oscillatory discharge. 
By using the blue tube as the source of light and the red tube 
as the absorption tube, the two being arranged so as to be illu- 
minated simultaneously, it might be found that the red tube had 
the power of absorbing, to some extent, the blue radiations from 
the other. 
Ihardly think results would be obtained, but the experiment 
seems worth trying. 
A picture taken by Mr. H. B. Lefroy, of Toronto, sent to me 
by Mr. Lumsden, Secretary of the Astronomical and Physical 
Society of Toronto, has some very curious appearances. There 
is an exceedingly brilliant flash running down the centre of 
the plate, illuminating the sky quite brilliantly in its neighbour- 
hood. In its immediate vicinity, though not joined to it in any 
way, are innumerable dark, thread-like markings, which in places 
seem to cross each other, forming meshes. Mr. Lumsden assures 
me that the testimony of all photographic experts who have seen 
the plate is to the effect that markings of that description could 
only be produced in the exposure—that is, they are not due to 
faults in the film or the results of imperfect development. The 
fact that they are found only in the immediate vicinity of the 
bright flash is additional testimony in the same direction. These 
markings are wholly different from any that I have seen, not 
having the form of branched flashes. Something in their re- 
semblance to photographs of sound-waves started by a spark, 
which I have recently made (see P2z/. A/ag. for August), sug- 
gested to me that they might possibly be due to the illumination 
of the sound-wave due to a powerful discharge by a second dis- 
charge. Under ordinary conditions, that is, with a uniformly 
illuminated background, such waves would of course be in- 
visible, but conditions might possibly arise due to the proximity 
of black clouds under which they might show—a sort of 
“Schlieren Methode” on a large scale. I have not attempted 
yet to plan an arrangement of clouds, which, by acting as screens 
to light coming from certain directions, might render visible a 
region of the air in which the optical density underwent a rapid 
change. 
Mr. Lumsden’s picture shows very black clouds irregularly 
distributed and in close proximity to the flash. 
