268 



NATURE 



[January i8, 1894 



involved in the introduction of an analyser in front of 

 the lens of the camera. 



Thei'e is still another method, unquestionably the 

 most simple one, and the one which, at the same time, 

 gives the best results : it is the employment of coloured 

 screens. In front of the lens of the camera is placed a 

 a screen which transmits yellow and green rays, 

 but is opaque to blue and violet rays. The light of 

 clouds is rich in yellow and green rays ; hence a large 

 proportion of it is able to traverse the screen, and act 

 upon the photographic plate, while, on the other hand, 

 the blue background of the sky emits very little yellow 

 light ; in fact, the proportion of rays of this refrangibility 

 decreases as the blue colour increases in depth, so its 

 action upon the sensitive film is considerably diminished 

 or altogether obviated. The only inconvenience of this 

 method is that yellow and green rays have very little 



i'lG. 2.— Cirrus cnc C'rcc-cuir-u'us (Feliuary i;, 1193.) 



action upon an ordinary photographic plate. Under 

 these circumstances it would be necessary to give a very 

 long exposure, which is impossible in cloud photo- 

 graphy on account of the movements of the objects 

 and the rapid changes of form. It is probably for 

 this reason that coloured screens, which w( re adopted 

 in the earliest stages of cloud photograjhy, appear 



NO. I 264, VCL. 49] 



now to have almost been abandoned. But this diffi- 

 culty has been practically overcome by the produc- 

 tion of the orlhochromatic or isochromatic plates of 

 commerce, which are sensitive to yellow [[and green 

 light. 



iM. Angot finds that the best brands of plates for use in 

 cloud photography are the Lumiere orthochromatic and 

 Edwards' isochromatic. Other brands have been tried, 

 but none gave better results than these. As to the yellow 

 screen, the best is obtained by placing a cell having 

 parallel faces, about five or seven millmietres apart, in 

 front of the lens, and filling it with an almost saturated 

 solution of bichromate of potash to which a few drops of 

 hydrochloric or sulphuric acid have been added. A mix- 

 tuie of saturated solutions of bichromate of potash and 

 copper sulphate in the proportion of three of the former 

 to one of the latter may also be employed. In either 

 case the cell is hermetically sealed, and 

 it can easily be fixed in front of the 

 lens or behind it in the bellows of the 

 camera. 



Evidently it would be simplest to use a 

 screen of coloured glass, and, as a matter of 

 fact, certain glasses give as good results as 

 the cell containing one of the above-men- 

 tioned solutions. But most yellow glasses 

 are quite inadequate for the purpose. It is 

 to be hoped that coloureJ-glass manufac- 

 turers will soon make a glass which will 

 transmit exactly the same rays as the solu- 

 tions. It will be a good thmg to have a 

 series of glasses of graduated tints ; the 

 clearest to serve for very bright white clouds 

 standing ou: boldly upon a fine blue sky, 

 while the darkest could be used for faint 

 clouds when the blue colour of the sky is not 

 so pronounced. The time of exposure must, 

 of course, be increased as the glass used is 

 increased in tint. 



The two illustrations here given are re- 

 duced copies of two of M. Angot's negatives. 

 The originals are eighteen centimetres long 

 by thirteen wide. 



Fig. I was obtained on March 31, 1892 ; it 

 shows some patches of cumulus cloud, and 

 an extremely remarkable sheaf of cirrus 

 which preceded a violent storm by two hours. 

 The second illustration (Fig. 2) shows a form 

 intermediate between cirrus, properly so 

 called, and cirro-cumulus, observed on Feb- 

 ruary 19, 1893. Both these pictures were 

 obtained by means of Lumiere orthochro- 

 matic plates, with a cell containing a solu- 

 tion of potassium bichromate and copper 

 sulphate, and a wide angle lens having a 

 focal length of o-i6o metres. The aperture 

 was cut down by means of a diaphragm of 

 about one-twentieth the focal length, and 

 the time of exposure for Fig. i was three- 

 quarters of a second, and one-half a second 

 for Fig. 2. The usual developers may be 

 employed, but pyrogallic acid was used by 

 M. Angot on account of the latitude of 

 exposure it permits. 



As photography is being widely used in 

 the future to increase our knowledge of 

 clouds, it is recommended that the date and hour of ex- 

 posure be written upon each picture. M. Angot's photo- 

 graphs are a sufficient testimony of the excellence of his 

 method of work, and their multiplication in different 

 parts of the world would considerably extend our 

 knowledge of cirrus clouds, and very probably prove 

 of use in forecasting weather. 



