ON PHOTOGKAPHT OF METEOROLOGICAL PHENOMENA. 135 



committee can form a compreliensive collection, it seems tlaat the accurate 

 registration of cloud photographs must be left in abeyance. Perhaps by 

 this time next year, if they are permitted to continue their work, some- 

 thing of the kind may be found practicable by referring other photo- 

 graphs to types in their own collection. 



Another important collection is in the possession of the chairman of 

 your committee. An early opportunity will be taken for the tabulation 

 and registration of its contents. 



Methods of Cloud Photography. 



Specimens of cloud photographs have been received illustrating 

 several methods. 



1. By the courtesy of the Kew Committee of the Royal Society six 

 specimens of the photographs taken under their direction have been 

 placed at the disposal of your committee. These have been taken in a 

 special form of camera provided with a rotating shutter, the opening of 

 which can be varied at pleasure. The exposure given is a fraction of a 

 second, and the plates ai-e of the rapid gelatine bromide type. So far as 

 definition is concerned, these pictures leave little to be desii-ed. 



2. Mr. A. E. Western sends one printfrora a negative taken on Edwards' 

 medium isochromatic plate, and two taken with Carbutt's orthochro- 

 matic celluloid films, in all cases after placing a sheet of pale yellow glass 

 in front of the lens. The definition in all three is good, but the type of 

 cloud is one which is easy to photograph, and it does not yet appear 

 whether the method is of very much value for thin cirrus clouds. 



3. The secretary to your committee has made a careful trial of two 

 other methods. 



The first consists in placing a plane mirror of black glass in front 

 of the lens, so that the plane of its surface makes an angle of about 

 33° with the axis of the lens. This method has been theoretically 

 described by Dr. Riggenbach in a paper read before the Royal Meteoro- 

 logical Society on November 21, 1888. It is supposed to depend on the 

 extinction of the polarised component of the light from the blue sky. But 

 in practice it is found that the mirror is of great advantage, altogether apart 

 from any polarisation. It diminishes the brilliancy of the whole illumi- 

 nation, so that it becomes easy to time the exposure correctly. By this 

 means it is found perfectly simple to get good negatives of even very 

 delicate cirrus clouds on any of the ordinary brands of dry plates. The 

 negatives frequently require intensification in order to bring out all possi- 

 ble detail, and it seems that transparencies on glass or prints on bromide 

 paper are to be preferred to ordinary silver prints. 



The second device which has been tested is the employment of slow 

 plates. Very satisfactory results have been obtained by exposing in the 

 camera some of the plates prepared for transparency work by Mawson 

 and Swan. This method has not been tried so thoroughly as the other, 

 but enough has been done to show that it may be recommended. 



The lens used in both cases was an Optimus rapid i-ectilinear with a 



stop Z With ordinary plates and the black mirror the exposure varied 



from about a tenth to half of a second, and with the transpai-ency plate 

 about twice or three times as long. 



The experiments wiU be continued throughout the summer, and your 



