PHOTOGRAPHY AND METEOROLOGY. 
163 
Hence we have issued some simple instructions as a guide to any 
who maybe willing to co-operate with us (see p. 167), and also some 
printed forms upon which certain facts relating to each photograph 
should be recorded. 
Perhaps the most important objects to be photographed are 
clouds. There are probably few owners of a camera who have not 
tried to obtain pictures of these beautiful objects; but they are by 
no means easy to take. The blue light of the sky affects a sensitive 
plate almost as much as the white light from the cloud. The 
exposure must therefore be very accurately judged, or the result 
will be unsatisfactory. I have read minute directions for cloud- 
photography in which you are told to stop down your lens to a 
small aperture, and then give an exposure of one second or more. 
This will do well enough if the clouds are stationary or nearly so, but 
as a rule they are not only in rapid movement, but also undergoing 
changes of shape and size far greater than is commonly supposed. 
Hence exposure must generally be limited to a fraction of a second 
or the detail will be greatly marred. 
The stop of the lens must not then be small, and exposure must 
be very accurately judged, or the image will be too faint. Heavy 
clouds of the type of shower-clouds are the easiest to take in full 
daylight, and any kind of cloud which stands out dark against a 
background of either an evening or early morning sky can be 
secured without difficulty. But the crux of cloud-photography is 
to get good pictures of thin white clouds, such as cirrus, in the full 
glow of sunlight. It can be done, no doubt, with sufficient practice, 
but most people would fare much as I used to do myself, securing 
one respectable negative in perhaps twenty attempts. 
Hence various devices have been resorted to. Some persons 
have tried a slow plate; others have placed a sheet of glass, tinted 
pale yellow, in front of the lens. I am not able to speak of either 
of these methods from personal experience, but the Committee 
hopes to receive examples of the results of each process, so that an 
opinion may be formed of their respective merits. 
There is, however, a third method by which I know it is quite 
easy to secure good results even with the thinnest kinds of clouds. 
This is by fixing a mirror of black glass in front of the lens so that 
the plane of glass makes an angle of about 33 degrees with the axis 
of the lens. The theory of the method is simple, and, as it has 
been explained by Dr. Biggenbach in the ‘ Quarterly Journal of 
the Boyal Meteorological Society ’ for October, 1887, I need not do 
more than give a very brief statement. The blue light of the sky 
is partially in the condition known as polarised, and a mirror of 
black glass upon which those polarised rays fall at a certain angle 
cannot reflect them. The object of the mirror is to stop these rays. 
Meanwhile the light from the clouds is not polarised, and is there¬ 
fore reflected. Theoretically the mirror should be of great value 
for a cloud at right angles to the sun, and of little use for others 
close to the sun or nearly opposite ; but as a matter of fact it serves 
a purpose quite apart from any question of polarisation. It 
