78 REPORT — 1892. 



Mr. Bateman's photographs the narrow ribbon structure in each flash is 

 not more pronounced in the moving photograph than in the one which was 

 stationary. 



The Royal Meteorological Society has received several new photo- 

 graphs of lightning. They all show the narrow ribbon structure ; one 

 shows reduplication of the images of some chimneys as well as the flash, 

 while another shows a bright flash and several dark ones. Of this last 

 Mr. Robert Law, who took the negative at Melbourne, remarks that there 

 were two flashes, the second reversing the image of the first. 



Methods of Cloud Photography. 



The information at the disposal of your Committee does not, as yet, 

 seem sufficient to enable them to pronounce definitely in favour of any 

 one method as the best. They have made some progress in ascertaining 

 the methods adopted abroad, and also in experimental work at home. 

 The subject divides itself naturally into two sections, dealing respectively 

 with cumulus or heavy clouds, and with cirrus or other light clouds. 



Cumulus. — With all heavy clouds it is certain that admirable results 

 may be obtained with a little practice in adjusting the stop and length of 

 exposure. So far as the quality of the results obtainable is concerned, 

 there does not seem to be any manifest advantage in the use of a coloured 

 screen, of a black mirror, of specially slow, or of orthochromatic plates. 

 Nevertheless, it is quite certain that results of a given excellence are 

 more easily obtained on a slow plate with a strong developer considerably 

 restrained. It is equally certain that the use of the coloured screen, or 

 of the black mirror, renders the process easier still. 



With correct exposure and careful development it should not be 

 necessary to resort to intensification of the image. If some such treat- 

 ment should be required, inexperienced observers should be warned that 

 in the use of mercurial salts it is well to keep the plate in constant 

 movement while in the mercury bath, in order to avoid undue granulation 

 of the image. 



Cirrus. — Those who have made a special study of the photography of 

 thin clouds appear to be agreed that in order to bring out all the details 

 of their structure some special device must be adopted. 



By extremely nice adjustment of the exposure and subsequent intensi- 

 fication of the image, very fair results can be sometimes obtained ; but the 

 process is difficult, and only practicable in experienced hands. 



Dr. Riggenbach, who first described the black mirror device, recom- 

 mends that exposure should be so arranged that the sky leaves practically 

 no impression on the plate, while the thin image of the cloud must be 

 intensified by means of Schlippe's salt (sulphantimoniate of soda) . 



M. Angot, in a report presented to the Meteorological Society of 

 France, remarks that a black mirror is only advantageous when the 

 cloud is about 90° from the sun. In theory this is of course correct, 

 but the Secretary to your Committee has found that there is a manifest 

 practical advantage in its use for all parts of the sky, including even the 

 immediate neighbourhood of the sun itself. M. Angot then goes on to 

 say : ' The best results are obtained by coloured screens, yet the ordinary 

 screens are insufficient. The following formula, due to M. Leon Vidal, 

 gives every satisfaction. In a small glass trough with parallel faces 

 there is placed a solution made with the proportions — sulphate of copper 

 175 grams, bichromate of potash 17 grams, sulphuric acid 2 cubic 

 centimetres. These are dissolved in 100 to 500 cubic centimetres of 



