January i8, 1894] 



NA TURF. 



267 



still look back on the work accomplished by Prof. Rudolf 

 Wolf as a germ from which their work had developed, and 

 as a monument of pains and industry. In his death, 

 besides a true friend, we lose a thorough devotee to 

 science, and we can ourselves mourn with his friends who 

 say, " Und heute stehen seine Freunde aus alien Gauen 

 des \'aterlandes trauernd am offenen Grabe, der Erde die 

 sterbliche Hlill? eines Mannes ubergebend dessen geis- 

 tige Grdsse, personliche Bescheidenheit und herzlichste, 

 oft aufopfernde Liebenswiirdigkeit alien die ihn gekannt 

 haben unvergeszlich bleiben wird " W. J. L. 



CLOUD PHOTOGRAPHY. 



T A NATURE recently printed an article by M. A. 

 -^ Angot on the methods he has been employing 

 in order to obtain the excellent photographs of clouds 

 exhibited at the Paris Physical Society at 

 the beginning of last year. The following 

 is a free translation of the article : — 



It is well known that ordinary photo- 

 graphic plates are most sensitive to blue and 

 violet rays ; hence the blue background of 

 the sky acts, in general, nearly as much 

 upon the plates as the white parts of clouds, 

 which are thus rendered almost or entirely 

 indistinguishable upon the photograph. It is 

 possible, however, easily to obtain views 

 of an interesting effect when, on a back- 

 ground of blue sky, large clouds pass be- 

 fore the sun. The edges of the clouds 

 are then lit up to such an extent that they 

 make much stronger impressions upon 

 the sensitive plate than the sky itself; the 

 remainder of the cloud is, on the contrary, 

 dark, grey or black, and does not come out 

 as well as the sky. To obtain an accurate 

 picture under these conditions it is neces- 

 sary to develop with great care : or better, 

 to use a dilute pyro developer — a few drops 

 of bromide of potassium solution and very 

 little pyro to begin with ; the development 

 is then slowly carried on with the addition 

 of carbonate of soda, and pyro is only added 

 again towards the end if the plate lacks 

 clearness. 



This method ceases to give good results 

 when it is applied to ordinary clouds, and 

 becomes altogether useless for cirrus clouds. 

 But these are precisely the clouds the study 

 of which is most interesting ; they are com- 

 posed not of water vapour, but of ice- 

 needles ; and their forms and movements are 

 closely connected with changes of weather. 

 Cirri are the most difficult to photograph 

 because, being farther from us than other 

 kinds, they are less brilliant ; and further, 

 when they are seen, the sky is very frequently 

 pale blue in colour, or covered with a 

 milky veil, which diminishes the contrast. 



Numerous plans have been proposed to 

 photograph cirrus clouds. The first consists 

 m photographing from the summits of high 

 mountains, but that method is not within 

 the reach of everybody. At such places the 

 sky is, in general, much darker, and the 

 clouds are better seen upon the background, so that 

 excellent photographic images can be obtained with- 

 out special devices. Another method has been pro- 

 posed by Prof. Riggenbach, and appears to have 

 some advantages. It consists in photographing the 

 sky, using a diaphragm so small and giving an ex- 

 posure so short that only a trace of the cloud-image 



NO. T264, VOL. 49] 



appears after development. The plate is then intensified, 

 and the image brought out by means of bichloride of 

 mercury and sodium sulpho antimoniate. This method, 

 however, has little to commend it. In the first place, 

 intensification is always inconvenient and destroys details, 

 and further, the sodium salt very rapidly deteriorates, so 

 there is always a risk of the plates being spoiled by 

 becoming a very intense yellow colour, or being covered 

 with a metallic deposit. 



Prof. Riggenbach has suggested another and abetter 

 method, which is found to give excellent results. The 

 method is based upon the fact that the blue light of the 

 sky is partially polarised, whilst the light of clouds does 

 not possess the same property. If, therefore, a con- 

 venient analyser (a Nicol's prism or black glass inclined 

 at 55 ) is placed in front of the lens of the camera, only 

 a portion of skylight is obtained, while the light of the 



Fig. I. — Cirrus Cloud prececiing a Storm (March 31, 1892). 



clouds remains unaltered, and the increased contrast 

 renders it an easy matter to obtain a good picture. But at 

 the same time, this method possesses inconveniences. 

 The proportion of polarised light is far from being the 

 same in all parts of the sky ; hence it is not possible to 

 photograph clouds in any direction. Moreover, many 

 photographers object to the complications which are 



