FEBRUARY 4, 1897 | 
NATURE 
20 
heard in the past, we know not with what truth, of art- | the camera, scientifically used, that 
| best results. 
fully-placed pieces of cotton-wool on the printing frame, 
and of other devices, which, by judicious handling, have 
been made to give an appearance remotely resembling 
that of natural clouds, and that competent judges have 
been deceived-by these means. A great authority on 
photographic reproduction has laid it down as a rule | 
that the same “sky” should not be printed’on more than 
one picture ; and that such advice should be considered 
necessary, shows the length to which ingenious fraud has 
been carried in this matter. And yet it might be thought 
that if accurate reproduction of cloud-forms was at- 
tempted anywhere, it would be found in photographs 
of landscape. Some of the reasons for its non- 
appearance, very well known to practical photo- 
graphers, have recently been discussed in NATURE (No. 
1367), and valuable suggestions made to overcome these 
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difficulties. © The photographer does not always wish to 
reproduce the actual state of the sky at the time his 
photograph was taken. He thinks he can produce a 
better. artistic effect by employing clouds of his own 
manufacture ;'but, apart from this, there is an inherent 
difficulty in- obtaining ~a- «satisfactory negative-of the 
lighter forms of cirrus and cirro-cumulus. The blue 
colour of the sky has ‘practically the same chemical 
action on the sensitised plate as that exercised by the | 
white colour of these fleecy clouds, and the contrast on 
which the photographer relies for his effects is wanting. 
The rules of procedure there given have resulted in some 
excellent representations, of which specimens will be 
found in the recently issued International Cloud Atlas, 
a work that may possibly revolutionise our methods of 
cloud observation. 
Although the processes of photography have been 
sadly abused, it is undoubtedly to the employment of | 
NO. 1423, VOL.°55] 
we must look for the 
Every day sees these results accumulating, 
and, as a necessary consequence, the introduction of 
greater uniformity in the classification and nomenclature 
of cloud observations. Also, greater and more frequent 
use suggests numerous devices to the expert, by which 
he may win more trustworthy pictures of the lighter 
forms. 
It is now possible to reproduce very light cirro-cumulus 
clouds, and though some of the delicacy of the original 
may be lost in the method of printing, sufficient detail 
remains to enable one to judge of the success that attends 
the processes that Prof. Riggenbach, of Basle, and others 
have successfully advocated. Prof. Riggenbach avails 
himself of the fact that, while the light from a cloud is 
only slightly polarised, the light from the blue sky is 
much more so, especially at points which are go° from 
Fic. 1.—Cloud, photographed at an altitude of 2500 m, 
the sun. By employing a Nicol prism, therefore, the 
skylight may be darkened to a very considerable extent, 
while the fleecy filaments of the cloud will come out with 
greater sharpness and distinctness. A dark mirror may 
be employed instead of the Nicol prism, and a still simpler 
means is to use the still surface of a lake as a polarising 
mirror. When clouds have an altitude of about 37°, and 
| differ in azimuth from the sun by about go”, they can be 
photographed in this way with ease and truthfulness. 
Another method which secures admirable results, though 
it may not be at the disposal of every one, 1s to photograph 
the clouds directly, at a considerable elevation above the 
sea-level. Here, on the top of a mountain summit, the 
sky appears much darker than in the plain, caused 
probably by the absence of scattered light from dust 
particles, which are more numerous in the lower strata. _ 
The observer, too,, has the additional advantage of 
lessening the distance between himself and the cloud 
