4 J. Trowbridge — Spectra from the 



dark lines instead of bright lines. Moreover, what are ap- 

 parently the strongest bright lines of the water vapor spec- 

 trum are not reversed. There is a selective reversibility which 

 arises with high temperatures, and is made evident by the faint 

 background of the continuous spectrum. 



This fact seems to me to be of great importance in the appli- 

 cation of photography to the study of celestial phenomena, for 

 reversal of spectrum lines do not necessarily indicate reversing 

 layers of cooler gas, and may be a photochemical action of the 

 silver plate. 



One immediately thinks in this connection of the phenome- 

 non of dark lightning or the Clayden effect, and of the sug- 

 gestive experiments of Professor Nipher.* Spectrum analysis, 

 however, reveals a selective reversibility, which shows that the 

 subject of photochemistry must be carefully studied before we 

 can interpret properly the records of photography in spectrum 

 analysis. 



There are doubtless many states of vibration even in the 

 actinic portion of the spectrum which are not recorded by the 

 silver salts, for a selective reversing action may obliterate or 

 prevent a permanent record. 



I have obtained this reversing action with different emul- 

 sions on glass and also on celluloid films. The strongest 

 reversals are at wave lengths 4227, 3930, 3965. There is also 

 a reversed band between wave lengths 4315 approximately 

 and 4285, and a faint reversal at wave length 3953. Rever- 

 sals are often seen on the negatives which disappear in the 

 fixing bath.f 



In this investigation ten thousand storage cells were em- 

 ployed to charge a glass condenser of '60 microfarads. The 

 charge was sent through Geissler tubes having an internal bore 

 of one millimeter; the length of the capillary of this diameter 

 was from eight to ten centimeters. The tubes were filled with 

 apparently dry hydrogen at a pressure of approximately •l"'"'. 



I have reached a limit in submitting gases in glass tubes to 

 powerful electric discharges, and am now turning my atten- 

 tion to the subject of quartz tubes in the hope of obtaining 

 these in a suitable form for containing rarified gases. 



In a previous paper,:j: I have expressed my conviction that 

 the four-line spectrum observed in the protuberances of the 

 sun is an evidence of the dissociation of water vapor in the 

 sun's atmosphere, and an evidence therefore of the presence 



* Transactions Academy of St. Louis, vol. x. No. 6, On certain properties 

 of light struck photographic plates. 



f In this connection the following paper is significant : Disappearance of 

 images on photographic plates. William J. S. Lockyer, Nature, Jan. 17, 

 1901. 



X This Journal, vol. x, Sept. 1900. 



