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LX^II. The Clayden Effect and Reversal of Spectrum Lines. 

 By Prof. E. W. Wood * 



IN the July number of the Philosophical Magazine Professor 

 Trowbridge publishes a series of photographs of the 

 spectra obtained by passing single powerful discharges 

 through Pliicker tubes containing hydrogen. Some of the 

 lines of the spectra appear reversed, which the author explains 

 by supposing a selective reversibility of the silver salts for 

 certain wave-lengths, expressing the opinion that the phe- 

 nomenon is of great significance in the application of 

 photography to astrophysics. 



" One immediately thinks," he says, " of the phenomenon 

 of dark lightning or the Clayden effect." 



Strong reversals were found at wave-lengths 4227, 3930, 

 and 3965. These are undoubtedly the calcium lines, which 

 almost always appear in the spectra of heavy discharges, 



It appears to me that the whole matter can be very easily 

 referred to the Clayden effect ; and I cannot see how the 

 phenomena have any bearing on the interpretation of astro- 

 physical photographic records. 



It will be remembered that Clayden showed that if the 

 image of a lightning flash or spark was thrown on a photo- 

 graphic plate, which was subsequently illuminated with 

 diffused light, the spark-images were reversed. That this 

 was not ordinary reversal due to extreme brilliancy he 

 proved by reversing the order of the two exposures, in which 

 case there was no reversal. 



He was unable to obtain the effect with any other source 

 of light than the electric spark, and referred the phenomenon 

 to some peculiarity of the light originating in an electrical 

 disturbance. 



In repeating and extending the work of Clayden I showed 

 (Nature, Nov. 30, 1899 ; Science, Nov. 17, 1899) that the re- 

 versing power extended throughout the entire spectrum ; in 

 other words, that it was not selective, and that it could be 

 obtained with the light from the crater of an arc-lamp, and 

 from the incandescent lime of the oxy-hydrogen light. It 

 was also shown in the same paper that the time element was 

 the all-important factor, flashes of light of a duration of ^~ 

 of a second or longer not reversing under any circumstances. 



The effect of a " light-shock " of a very brief duration, 

 say less than ^r^ sec -? * s *° diminish the sensibility of the 



* Communicated bv the Author. 



