'25S Prof. R. W. Wood on Screens 



of iodine in bisulphide of carbon, must have wished that we 

 possessed a screen, opaque to visible light and transparent to 

 the ultra-violet. 



I have recently succeeded in making a screen quite trans- 

 parent to these radiations, though a gas flame cannot be seen 

 through it. By combining it with a large condensing-lens 

 and an arc-lamp, it is possible to form a dark focus of ultra- 

 violet light in which a lump o£ uranium nitrate glows with a 

 vivid green phosphorescence like a great emerald. 



Besides giving us the means ol" performing a most beautiful 

 lecture experiment, these screens make it possible to photo- 

 graph the ultra-violet lines in grating spectra of higher 

 orders than the first, entirely uncontaminated by the visible 

 radiations which overlie them. Other applications at once 

 suggest themselves, such as the complete removal of the 

 highly actinic blue and violet rays, in certain investigations 

 of the ultra-violet region where the long exposures necessary 

 are apt to produce fogging of the plates. It seems quite 

 possible too, that photographs of the moon, planets, and 

 nebula? taken by means of ultra-violet light may furnish. 

 valuable data, as I shall attempt to show at the end of this 

 paper. 



The substance which has made possible the production of 

 such a screen is nitroso-dimethyl-aniline, the remarkable 

 optical properties of which I have already alluded to in a 

 previous paper. As I have already said, a prism formed of 

 this substance yields a spectrum about 30 times as long as a 

 quartz prism of the same angle, the dispersion resembling some- 

 what that of selenium. I was of the opinion that the absorp- 

 tion, which commences at about wave-length '0005, would 

 increase continuously from this point down to the end of the 

 spectrum, as was found to be the case with selenium. On 

 commencing a study of the absorption, however, I was 

 astonished to find that it ended abruptly a little beyond the 

 H and K lines, and that from this point on, the substance was 

 transparent even down to the last cadmium line, of wave- 

 length «0002. 



It at once occurred to me that if some substance or sub- 

 stances could be found absorbing the red, yellow, and green, 

 and transparent to the ultra-violet, we could, by combining 

 them with the nitroso compound, produce the long-sought 

 screen. 



Very dense cobalt glass, coated with a thin film of gelatine 

 lightly stained with the nitroso, was found to be transparent 

 only to the extreme red and the ultra-violet, and the red was 



