Clayden Effect and Reversal of Sjiectnun Lines. 607 



plate, so that a subsequent exposure does not produce so 

 intense an image as it otherwise would. 



The two things necessary for a Clayden reversal are, first, 

 a light -shock o£ great intensity and exceedingly brief 

 duration : and, secondly, a subsequent illumination by a 

 feebler light of longer duration. It is not difficult, it seems 

 to me, to see how both of these conditions could have been 

 fulfilled in Professor Trowbridge's experiments. The heavy 

 discharge produced a bright -line spectrum of sufficient 

 intensity and sufficiently brief duration to give the necessary 

 light- shock to those portions of the plate on which the images 

 of the bright lines fell. 



The heat of the discharge raised the inner wall of the 

 tube to incandescence, superposing a continuous spectrum 

 of much longer duration on the bright-line spectrum already 

 impressed. The bright lines would then come out reversed 

 exactly as they did in the experiment which I described in 

 : Nature/ 



Professor Trowbridge showed me some of the tubes used 

 in the experiments, and the badly corroded inner surface 

 indicates that it must have been raised to an exceedingly 

 hio-h temperature. A brief phosphorescence of the gas 

 following the discharge may have helped in the production 

 of the continuous spectrum, though I am inclined to refer it 

 chiefly to the incandescence of the glass surface. 



I feel confident that a photograph of one of these discharges 

 with a revolving mirror or moving plate would reveal the 

 dual nature of the illumination. 



One other point requires mention. Professor Trowbridge 

 states that the strongest bright lines are not reversed, and 

 that there is therefore a selective reversibility. That the 

 strongest lines should not appear reversed is precisely what 

 we should expect; for, as I showed in the experiments referred 

 to, if the initial illumination is too intense reversal does not 

 take place, i. e. the light- shock must not be too heavy. 



By taking a series of photographs of very bright sparks, 

 with different stops in the lens I found that only the images 

 obtained with the small and medium stops came out reversed. 

 It seems probable that the calcium lines in Professor Trow- 

 bridge's experiments had the requisite intensity for reversal, 

 while the other lines were too bright. If this explanation of 

 these pseudo-reversals be accepted, it does not seem to me 

 that they can have any bearing on stellar photographs, for it 

 is difficult to imagine how the dual illumination could be 

 produced. 



