210 



THE N RAYS OF M. BLONDLOT. 



observer, though at considerable distance from the tube, the brightness of the phos- 

 ]>liorescence is seen to diminish, and ujion removing the obstacle the brightness 

 again increases. The only precaution which it is necessary to take is to employ a tube 

 only slightly phosphorescent, but it is advantageous to place behind it a black pajier, 

 so that the interposition of the screen produces no change whatever in the back- 

 ground against which one sees the tube. The variations of l)rightness are most 

 easy to observe near the boundaries of the luminous spot formed upon the black 

 background by the jihosphorescent l)ody, and when the N rays are intercei)ted these 

 contours lose their sharjmess and regain it when the screen is removed. Sometimes 

 the variations of brightness are not instantly recognized. Interposition in the path 

 of the beam of several sheets of aluminum, of cardl)oard, and even of a board of 

 oak more than an inch thick, does not prevent the effect, so that all possibility of 

 the action of any ordinary radiation is of course excluded. A thin sheet of water, 

 however, entirely arrests the rays, and thin clouds passing before the sun consider- 

 ably diminish their action. 



WAVK LENCTII OF THK RAYS IN gUESTION. 



M. Jilondlot, as we have seen, was at lirst inclined to think that his 

 rays belonged in the extreme infra-red spectrum, but more recently 

 he has described measures of their wave length b}' means of the diffrac- 

 tion grating which lead him to the opposite conclusion. He employed 

 a spectroscope with aluminum prism to separate the several different 

 species of N rays emitted by a Nernst lamp, and then estimated their 

 wave length by means of several different diffraction gratings having, 

 respectivel}^ 50, 100, and 200 lines to the millimeter. The following- 

 table contains the results of his measures: 



Thus it appeal's that the N rays belong far beyond the previously 

 studied ultraviolet, and have a wave length only one-tenth that of the 

 rays with which Doctor Schumann has been working with his vacuum 

 spectograph. It is somewhat extraordinary that the N rays should so 

 readily traverse thicknesses of the air and other substances, which 

 would entirely arrest the ultraviolet rays examined by Doctor Schu- 

 mann, ])ut, as is tht? case in other regions of the spectrum, it may be 

 that the air has here special bands of great absorption, in one of which 

 Doctor Schumann's vajs lie, and that beyond this region there are 

 other parts of the spectrum where the air is again transparent. Another 

 curious thing about the measures just given is that the aluminum 



