498 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1906. 



equilibrium in the vertical direction, while the latter 

 shows a similar effort in the horizontal direction. It is 

 the interaction of these primary processes in greater or 

 less degree which is responsible for the .several inter- 

 mediate types of clouds. 



The cxi.stence of cloud at any elevation tells us that 

 the atmosphere in that particular part is in a state o' 

 satiM'ation, and that the vapour is being conden.sed. 

 The form of the cloud also indicates to ns the condi- 

 tion.s under which this air space became saturated with 

 the C()ns<"f|ucnt formation of cloud, and hence .shows, (o 

 some extent, changes which are in progress in the upper 

 air which may affect the weather at the earth's surface. 

 The direction of motion of the cloud finally gives u.s ;i 

 good clue to the winds which are blowing iiigh uj) in the 

 air. 



While the eye can be devoted to note the direction, 

 the camera should be employed toi record the form. 



In cloud photography, the great desideratum is to 

 produce contrast and secure clear, well-defined nega- 

 tives with Ijrilliant high lights. The predominance of 

 blue rays in reflected skvlight is in many cases a great 

 dr.-nvback, since the photographic action of blue is .il- 



this process he obtains results which, as he says, are 

 "uniformly and continuously successful." 



From the above remarks it will be seen that there 

 are many ways and means of photographing clouds, and 

 very much depends on the operator. 



My own experience of the subject is very limited, in 

 fact, my attention was only attracted specially to the 

 subject during April of the present year. I may, how- 

 ever, preface the following description of the method I 

 employ by saying that my object was not necessarily to 

 obtain pictorial photographs, but negatives w'hich 

 would bring out well the forms and chrmges of both 

 dense and very thin clouds. In fact, I have not yet 

 gone to the length of using a wide angle lens, so 

 necessary for including a large held, as my intention 

 up to the present has been to try and secure satisfactory 

 photographs. The photographs which accompany this 

 article will serve to show some examples of various 

 kinds of clouds, which are representatives of types of 

 photographs, all of which are obtained without any 

 dilhculty. The camera which I use (.see Fig. i) is a 

 small 5 by 4 instrument, made i)y W'inkel, of Gottingen, 

 (iermanv, and is fitted with ;i Zi'iss lens of 4.4 inches 



Fig. I.— Cloud Photographic Equipment. 



most identical with that of white light. It becomes 

 therefore, almost a necessity tO' quench the blue of 

 the sky while preserving the light of the clouds. The 

 whole art of successful cloud photography lies in over- 

 coming this difficulty. The ways of doing this are 

 many, and the efficiency of the different methods em- 

 ployed varies to a great degree. 



One of these methods includes the use of a diaphragm, 

 a quick exposure, and an after intensification of the 

 negative. A more efficient method seems to be the em- 

 ployment of a coloured screen in front of the objective, 

 the exposure of the plate necessarily depending on the 

 colour and density of the glass or liquid employed. 

 In fact, there are many variables involved, not for- 

 getting the speed of the plate itself. Even when the 

 exposure has been made the question of development 

 arises. Some, in fact most, authorities advise a fairly 

 weak solution with a slow methodical development. 



Many workers prefer to rely on backed slow plates, 

 a full exposure and a properly restrained developer. 

 Mr. Clayden, to whom reference has above been made, 

 photographs his clouds by reflection from a black 

 mirror placed just in front of his object gl.ass; he em- 

 ploys slow plates, namely, photo-mechanical, and by 



focal length, and 0.75 inches aperture. I usually work 

 the diaphragm shutter, giving an exposure of about one- 

 twentieth of a .second, with the lens cut down tO' about 

 /8. 



Fixed on the front of the lens is a kodak glass 

 yellow screen, which increases a normal exposure ten- 

 fold. Although I ha\e not u.sed many different kinds 

 of plates, I find that the ortho-process plates of Messrs. 

 Wellington and \\'ard (Elstree, Herts), with speed 

 number H and D 80, give very satisfactory results, so 

 satisfactory, in fact, that I have not looked further 

 afield. 



Regarding development, I rm no advocate of the 

 slow process, and prefer the negative to be fully de- 

 veloped in about five minutes. The actual solutions 

 I employ are given below, and equal quantities of each 

 are used; sometimes three or four plates are developed 

 w'ith the same five ounces of developer. 



HYDROQUINONE DEVELOPER (MAWSON). 

 No. 1 SOLUTION. 



Hydroquinone 320 grs. 



Bromide of Potassium .. .. 320 ,, 

 Metabisulplii'.e of Soda .. .. 320 ,, 

 Water (distilled) 80 ozs. 



