Septejibek 1, 1899.] 



KNOWLEDGE. 



203 



an angle of ninety degrees from the sun, if the heavens are 

 scrutinized in the proper direction through an analyzer 

 (Nicol or black minor), a large proportion of the rays emitted 

 by the blue of the sky will be quenched, while the intensity 



Fig. 3.— Stratus, 1898, Xorember, 2d. 4h. 15m. 



of the clouds will not be very considerably attenuated. 

 This enhances contrast, and gives fine views. 



Another method of the same physicist's consists in 

 simply photographing the sky with a small diaphragm and 

 short exposure. This gives next to nothing after develop- 

 ment ; but an energetic intensification of the negative with 

 bichloride of mercury and sulpho-antimoniate of 

 soda, seems to give fair results. 



The most practical way, however, of quenching the 

 blue of the sky is that of placing in front of the 

 objective a yellow glass or solution contained in a 

 cell with parallel sides. This is the method adopted, 

 after many trials, by MM. Hildebrandson, Angot, 

 and Teisserenc de Bort, and, invariably, by the 

 present writers. 



Exposuies in cloud photography are usually very 

 short. According to M. Angot, of the Bureau 

 Central Meteorologique, who is a past master on 

 the subject, a long exposure is usually preferable to 

 ■A short one, because, after development and fixing, 

 it is easy to give a long-exposed negative the 

 necessary intensity by dipping it in hyposulphite of 

 soda dissolved in water at ten per cent., to which 

 should be added a solution saturated with red 

 prussiate of potash. The action of this liquid on 

 the image is a reducing one, by which the intensity 

 of the negative is diminished. A prolonged washing 

 ought to follow. I'>y this means M. Angot drew 

 excellent results from over-exposed plates, while the 

 intensification of under-exposed ones gave him only 

 bad negatives. 



Having undertaken at Juvisy, at M. Camilla 

 Flammarion's suggestion, and under his superinten- 

 dence, a continued photographic study of the optical 

 phenomena of our atmosphere in general, and of the various 

 forms of clouds in particular, the writers thought it might 

 interest the readers of Knowledge to give them a brief 

 account of the work done here in this line since the summer 

 of 1897. 



The apparatus used is an ordinai-y seven-inch by five-inch 



camera , to which can be adapted either an " orthoperiscopic " 

 object-glass by Derogy, 1-42-inch aperture, or a smaller 

 glass by Levy, 1-01-inch only, but giving a wider field of 

 view. Various coloured screens have been utilized 

 here : (") a yeUow glass ; (i) yellow gelatine ; 

 and ( (') a solution of bichromate of potash con- 

 tained in a cell with rigorously parallel sides. 

 It is not difficult to make yellow glasses. 

 Fixing with hyposulphite of soda a thin, un- 

 exposed, photographic plate, and then dipping 

 it into cbrysoidine dissolved in water, the film 

 will be tinted with the most exquisite yellow. 

 The intensity of the colouring -will, of course, 

 entirely depend on the quantity of chrysoidine 

 used. 



Convenient as the interposition of a mere 

 coloured glass is, the use of a yellow solution in 

 a glass cell has been found more advantageous. 

 The varying intensity of clouds requires different 

 shades of yellow. Thus, very light cu-ri on a 

 milky sky will not print their delicate filaments 

 on the plate without a strong >iellow screen, 

 whUe a bright cumulus, projected on a dark 

 blue sky, might be advantageously photo- 

 graphed through a very light yellow only. 



But, however eflicient the coloured screen, 

 contrast will be rendered still more prominent 

 by the use of isochromatic plates. The pan- 

 chromatic plates of Messrs. A. Lumiere et ses 

 Fils, of Lyons, have been invariably used by the present 

 writers. 



The photographs accompanying this paper exhibit the 

 chief forms of clouds, by giving Luke Howard's three 

 primary types and their compounds. 



A cirrus of the " feather " or " spray " type is shown in 



Fig. -1. — Small Cumuli durinj fine summer weather, I'Jd. lli. 5m., 

 1898; September. 



Fig. 1. A glance bestowed on this photograph will give an 

 idea of the action of the yellow screen ; the cloud was so 

 delicate to the naked eye, that its ramifications could 

 barely be discerned from the bright bluish-white back- 

 ground of the sky, and yet its slightest filaments can be 

 detected on the negative, where the contrast with the sky 

 is most marked. The cirrus in question was the first 



