ON PHOTOGBAPHS OF METEOROLOGICAL PHENOMENA. 79 



■water, according to the thickness of the trough and the results to be 

 obtained. The sulphate of copper arrests the red rays, and the bichro- 

 mate the blue and violet. The plates used were Luniere's orthochromatic, 

 and the exposure from '5 to "8 second.' 



Your Committee regret that they have not yet received any illustra- 

 tions of the results obtainable by the above means, but it is hoped that 

 an exhaustive trial of the method may be carried out in the course of the 

 coming year. 



The Secretary to your Committee has continued the comparative 

 trial of slow and ordinary plates with or without a black mirror. He 

 reports that ordinary plates and direct exposure may often give satis- 

 factory results when the background of sky is a clear deep blue. If, 

 however, it is at all hazy, the correct exposure becomes extremely diffi- 

 cult. With slow plates, however, such as Mawson & Swan's trans- 

 parency plates or photomechanical plates, it is fairly easy to obtain 

 results of considerable excellence. 



With the black mirror ordinary plates give excellent results ; but 

 here again, unless the clouds are moving with unusual rapidity, or unless 

 the light is very bad, there is a great advantage in the use of slow plates. 

 i- It is, indeed, easy to obtain a fairly dense image of any cloud, however 

 luminous and however thin, by the combined use of mirror and slow 

 plate. Such means give abundant detail and full gradation of light and 

 shade, even when the sun is actually in the field of view. Exposure 



would vary from about '2 to about -8 second with an aperture — . 



The developer used in all these experiments is the familiar formula 

 with pyrogallol and sulphite of soda considerably restrained. 

 * Special attention should be drawn to the admirable series of cloud 



studies presented to the Committee which were taken by Signor 



rMannucci at the Vatican Observatory under the direction of the Rev. 

 Padre Denza, S.J. These show what can be done by direct exposure, 

 Signor Mannucci recommending a slow plate for the more difficult 

 subjects. The pictures taken by the Secretary to your Committee in a 

 similar manner show the value of the slow plate and black mirror. 



Miscellaneous Photographs. 



With regard to miscellaneous photographs of meteorological interest, 

 your Committee regard with some satisfaction the number of pictures 

 they have been able to secure which show the violence and severity with 

 which the great blizzard of March 1891 visited the South-west of England. 

 They believe it is of great importance that fairly complete pictorial records 

 should be kept of all such abnormal events. 



They are also pleased to be able to report that several of the photo- 

 graphic periodicals have recently manifested considerable interest in the 

 work, one paper, the ' Practical Photographer,' having just offered a series 

 of prizes for the best meteorological studies ; and the editor has promised to 

 present any competing pictures to your Committee. Several photographic 

 societies are also taking the matter up, and your Committee hope that the 

 effect of such powerful aid may rapidly make itself felt, both by increasing 

 their collection and by adding to the number of contributors. 



In order to show the widespread interest already taken in the subject, 

 brief catalogues of the three principal collections are appended. 



In conclusion, your Committee ask to be re-appointed with a grant of 

 15?., in order to follow up properly what they regard as a satisfactory start. 



