136 EEPOET— 1891. 



committee hope that they will soon be in a position to decide which 

 method is on the whole most suitable for the purpose. The black glass 

 method has the one great advantage that it works well with the ordinary 

 plates, and as the mirror may be easily removed and replaced a few cloud 

 pictures may be taken during any photographic excursion without the 

 necessity of carrying slides charged Avith plates of little use for other 

 purposes. 



Photographs of Lightxixg. 



The registration of photographs of lightning is beset with difficulty, 

 just such as interfered with the description of clouds. A provisional 

 classification has been issued under the authority of the Thunderstorm 

 Committee of the Royal ]\Ieteorological Society. Thi.s, however, was 

 premature, and cannot be regarded as satisfactory. Hence your com- 

 mittee have turned their attention rather to the study of lightning than to 

 recording pictures of it. 



The phenomena accompanying electric discharges do not seem to have 

 been very perfectly studied, but certain facts are known, and photographs 

 of lightning and of electric sparks point to others. It seems, therefore, 

 that no classification can be generally accepted which ignores existing 

 knowledge of the connection between the electrical conditions and the 

 character of the discharge. 



The so-called black flashes have of course been disposed of. The experi= 

 ments described two years ago by the Secretary to your committee showed 

 that the appearance is due to reversal produced by some form of diffused 

 light having fallen upon the plate. This conclusion has been subsequently 

 confirmed by ^Ir. Shelford Bidwell, F.R.S., and again by Mr. Clayden in 

 the photograph numbered 2B. This was taken at Bath in the early 

 morning hours of June 25. After the flash had passed, the plate was 

 left exposed for a few minutes in the hojje that a second flash might 

 illuminate the same part of the sky. This happened, the lower part of 

 the field of view being brightly lit up by a flash which was itself hidden 

 in the clouds. Where the consequent glare crossed the undeveloped 

 image of the flash reversal has occurred, while no reversal can be detected 

 in the other portion. 



It will be noticed that this flash, like many others, shows a distinct 

 ribbon-like structure. The repeated occurrence of this phenomenon has 

 already given rise to considerable discussion, and Mr. W. Marriott and 

 Mr. Cowper Ranyard have attributed it to a movement of the camera 

 during the existence of the flash. Certainly many such photographs 

 have been taken in cameras held in the hand or on no very firm base. 

 Moreover, Dr. Hofl'ert's photograiDh, No. 1 B, shows this structure well in 

 the successive bright flashes. Nevertheless, it must be noted that in this 

 last case the camera was in rapid motion, and yet the ribbon-like struc- 

 ture is hardly more pronounced than it is in other pictures where any 

 accidental movement was presumably much less. Moreoverj the photo- 

 graphs Nos. 2 B and 3 B show this structure very plainly, though the 

 camera was standing on a steady support, and movement during the flaeh 

 was quite out of the question. 



Alternative hypotheses are that the aj^pearance is due to reflection 

 from the back of the plate or in the lens. If either view were true the 

 brighter parts of the flash should show the ribbon form the best, whereas 



