PHOTOGRAPHY TO METEOROLOGY. 
165 
Alps, might afford very useful information, for it would be highly 
interesting to see a series of views of the same ice-stream at 
different dates and in different seasons. Evidences of abnormally 
severe winters, such as the one from which we are just emerging, 
would also be of great value in the future. 
Perhaps the most fascinating of all branches of photography is 
taking pictures of lightning. Some years ago the Royal Meteoro¬ 
logical Society took up this work, and it now possesses a fine series 
of lightning photographs, copies from some of which I am enabled 
by the courtesy of the Society to show upon the screen. 
Many of the results recorded by these photographs coincided 
exactly with anticipation from the analogy between a lightning- 
flash and an electric spark. Others, however, were entirely unex¬ 
pected, the strangest being the occurrence of some flashes which 
show dark in the print. For some time the explanation of this 
was unknown. Most people believed it to be due to some form of 
photographic reversal similar to that which causes the image of 
any very bright object to print black. However, the flashes did 
not seem brilliant enough to act in such a way, so the point was 
a puzzle until I had the good fortune to light upon the proof that 
the darkness was due to reversal. I found that the latent image 
of a lightning-flash or electric spark which would develop normally 
may be reversed by exposing the plate before development to some 
diffused light. The glare from another flash or spark reflected 
from a white background such as a cloud can effect this reversal, 
but it seems essential that the diffused light should be subsequent 
of the impression of the latent image. My experiments have been 
fully confirmed by Mr. Shelford Bidwell, F.R.S., so that I have 
no misgivings about it. Nevertheless we should like to get two 
photographs of the same flash, one showing it bright and the 
other dark. 
Another strange thing was that many lightning-flashes seemed 
to be broad ribbons of light or a volley of parallel flashes following 
each other through a complicated series of twists and turns. What 
were these ? Mr. W. Marriott and Mr. Ranyard found that all, 
or at least most of them, had been taken in a camera held in the 
hand. No doubt those who took them relied upon the statements 
in all books on electricity as to the extremely short duration of a 
flash. I never had any doubt upon this point myself until I saw 
the photographs. Then came a photograph taken by Dr. Hoffert. 
He deliberately swayed his camera from side to side, and the 
resulting picture showed a series of flashes following one another 
along the same or a precisely similar path, while something kept 
shining all the time and produced broad streaks across the 
negative. 
Lightning is, then, certainly not always the single instantaneous 
flash it has been supposed to be. It often consists of a volley of 
sparks, and it is about those volley flashes that we want more 
knowledge. 
Nothing is easier than to photograph lightning provided a 
