THE N RAYS OF M. BLONDLOT. 209 



Other methods of recoj^nizing the rays were now introduced, for 

 M. Blondlot was led to inquire whether the sparking device acted as a 

 sign of their presence b}^ virtue of its electrical properties or by vir- 

 tue merely of its emission of light. Accordingly he used a small blue 

 flame instead of the device, and found with it also an increased lumi- 

 nosity when placing it in the focus of the rays. A little later he 

 found that phosphorescent substances, though not excited directly by 

 the rajs, yet if first made feebly luminous b}' ordinary" light were 

 raised to a higher luminosit}' when exposed to the N rays. In later 

 experiments it appeared that a surface feebly illuminated by reflected 

 light became brighter under the influence of N rays. Still more 

 remarkable, lie found that if the N rays fell onl}^ on the eye of the 

 observer, and not on the object observed, the latter was nevertheless 

 made to appear more luminous, though the N rays themselves produce 

 no sensation of light. Photography failed as a direct method of 

 observing the ravs, but he used it indirectly to note the increased 

 luminosity of the spark, the blue flame, or the phosphorescent surface 

 which was employed to recognize the presence of the rays. The 

 accompanying flgure, taken from the Comptes Rendus of February 

 22, 190-1, shows an example of this indirect photographic metliod. 

 Experiments with the most sensitive apparatus failed to record an}' 

 sensible heating produced by the N rays. 



M. Blondlot makes the following general rcMuark concerning the 

 observation of the N rays: 



The ability to recognize slight variations of luminous intensity varies very much 

 between different persons. Some see at the first glance, without any difficulty, the 

 augmentation which the N rays produce in the brightness (if a small luminous source, 

 while to others these changes are very near the limit which they can distinguish, 

 and it is only after some experience that they are able to be sure of having observed 

 the phenomenon. The feebleness of these effects and the delicacy of the observation 

 ought not, however, to arrest our study of these heretofore unknown radiations. I 

 have found recently that the Welsbach burner may be advantageously replaced as a 

 source by the Nernst lamp with no glass covering, for this latter gives forth the 

 N rays with greater intensity, and thus with a 200-watt lamp, for example, the phe- 

 nomena are so marked that they may be easily observed. 



N RAYS FROM THE SUN. 



The following simple experiment is given by the discoverer to show 

 the existence of N rays in the solar beam: 



A completely darkened chamber is furnished with a window exposed directly to 

 the sun's rays, and this window is closed by an oak shutter at least half an inch 

 thick, so that no ordinary light can possibly penetrate into the room. Behind this 

 shutter, at about a meter distance, for example, is placed a small glass tube contain- 

 ing -a phosphorescent substance — sulphide of calcium, for example — whicli has pre- 

 viously been exposed to light and become feebly luminescent. If, now, in the beam 

 of the sun, which we suppose to pass through the wooden shutter and fall upon the 

 phosi)horescent tube, we interpose a screen of lead, or even simply the hand of the 



