OcrosER 12, 1899] 
NATURE 571 
again confronted with many difficulties, for the action 
of light on the sensitive film is capable of giving us both 
bright and dark images, although the object photographed 
is bright. We have, therefore, to contend with reversals, 
double reversals, &c., and many as yet unknown factors. 
There is one point, however, that stands out foremost, 
and that is that the photographic plate has recorded 
many times dark as well as bright flashes ; but whether 
the dark flashes are due simply to some action relative 
to the sensitive film, or are actual images of real dark 
flashes, is the very question that has so recently been 
revived. 
What we really are greatly in need of is more data, 
and when a sufficient number of photographs of all kinds 
of lightning has been collected, more light will be 
thrown on this subject. Up to the present time, as 
each curious photograph of dark lightning was published, 
suggested theories as to the cause of the peculiarity of the 
flash have been by no means few in number, so that now 
the number of hypotheses equals, if not exceeds, that of 
the photographs examined. 
In a very interesting article in this journal (vol. 
xlii. p. 15i), which is an extract from a lecture on 
“Electrical Phenomena in Nature,” delivered by Mr. 
Shelford Bidwell at the London Institution, the so-called 
““dark flash” is referred to in these terms. 
“ It occasionally happens that, on developing a photo- 
graphic plate which has been exposed during a thunder- 
storm, the image of a lightning flash comes out black 
instead of white. . . . There is no need to discuss the 
several ingenious hypotheses which were suggested in 
explanation of the anomaly ; it is sufficient to say that 
the mystery was completely cleared upa few months ago 
by the experiments of Mr. Clayden.” 
As I have no reference to Mr. Clayden’s experiments 
at hand, I will quote from the above-mentioned abstract 
a brief summary of his hypothesis as described by the 
same writer. 
“Tf the lens of the camera be covered the moment 
after a flash has occurred the developed image will 
always come out bright, feebly or strongly, according to 
circumstances. If, however, the plate be exposed after a 
flash has acted upon it, either to the continued action of 
a feeble diffused light or to the powerful glare arising 
from one or more subsequent flashes, then on develop- 
ment the image of the original flash will probably come 
out black. The effect is therefore not a meteorological 
or physical one, but purely chemical. It can be obtained, 
not only with a lightning flash, but also with a machine 
spark, or even with an ordinary flame. It is merely 
necessary that the plate should be exposed to the action 
of a certain amount of light after it has received the 
impression and before development.” 
At the present time Mr. Clayden’s explanation may be 
looked upon as the most reasonable working hypothesis 
for future use. There is one crucial test which can be 
tried, which would settle once and for all its value. Un- 
fortunately, so far as I am aware, this test has not yet 
been made, and I propose (and I hope others will as 
well) under the next suitable conditions to make the 
attempt. It is simply this. Take two cameras, say A 
and B, and orient them both in the same direction 
towards the point where the same flashes will come in 
both fields of view. Expose A for say fifteen minutes 
to record all the flashes that occur during that interval 
(some of these on development should be é7zgh7, some 
dark). EExpose B for one flash only, preferably the 
first bright one which occurs during the exposure of | 
A; this should develop é7zght. Compare the same 
flash on both negatives ; that in A should be dark, 
that in B bright. If this be not the case, then I think 
the hypothesis breaks down. Perhaps this experiment 
may not be so easy to perform as it at first appears, 
for the difficulty lies in being able to catch one strong 
NO. 1563, VOL. 60] 
flash without exposing the plate to any light from other 
flashes which illuminate the sky, but are not in the field 
of view themselves. Several attempts by numerous 
observers would probably give us the information 
required. 
With the object of firstly contributing data towards the 
interpretation of this curious and interesting phenomenon 
as recorded by the sensitive; film, I give here some 
illustrations from 
absolutely funtouched negatives of 
Fic. 
Germany, in 1893. 
lightning flashes. All these reproductions are reduced 
about one-third. 
I may perhaps preface my descriptions of the photo- 
graphs by the remark that, having always taken a great 
interest In procuring lightning flashes by the aid of the 
camera, I have never, until this year, been fortunate 
enough in securing records of dark flashes. I have 
always previously exposed my plates or films for periods. 
of fifteen minutes or more, depending on the strength 
and nearness of the storm. This fact at first suggested to 
me the idea that dark flashes might after all be real, but 
Fic. 2.—Showing dark (a and B) and bright (c and p) flashes photographed 
at Westgate-on-Sea on August 5, 1899. 
restricted to certain kinds of storms, the special pecu- 
liarities of which I cannot state. 
Fig. 1 is a type of several negatives I have secured 
previous to this year, and although the exposure was. 
twenty-five minutes in length, an examination of the 
negative shows absolutely no trace of any dark flash. 
The photograph, which was taken at Gottingen in North 
Germany, Is interesting on account of the fine flash (a) 
which is traversing the air in a nearly horizontal direction 
and without any branches or ramifications. In the right- 
t.—Lightning flashes taken during a thunderstorm at Gdttingen, *y 
