JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII JANUARY 4, 1917 No. 1 



PHYSICS. — Luminescence measurements. 1 N. E. Dorsey, Bu- 

 reau of Standards. 



Although the excitation of phosphorescence by the radia- 

 tions from radioactive substances was early observed it appears 

 that no quantitative measurements of the luminosity thus pro- 

 duced were published until 1910. In that year Marsden 2 pub- 

 lished a series of observations which showed that the lumines- 

 cence of certain materials (zinc sulphide, willemite, and barium 

 platinocyanide) hermetically sealed in tubes containing radium 

 emanation decreased much more rapidly than the emanation 

 decayed, and that the rate of decrease increased with the amount 

 of emanation initially present. He reported only relative values. 

 At the same time Rutherford 3 advanced a theory to account 

 for these phenomena, and showed that it agrees with Mars- 

 den's observations, within the limits of experimental error. 



No later measurements of such luminosities have apparently 

 been published, although the subject is one of considerable 

 scientific interest. Latterly the subject has become of much 

 practical importance, owing to an increasing use of self-lumi- 

 nous preparations containing radioactive excitants. Besides 

 being used for the illumination of watch dials, push buttons, 

 etc., these preparations have a wide field of usefulness in war 

 appliances. 



In the study and testing of these preparations we are con- 

 fronted with the problem of measuring surface luminosities of 



1 A contribution from the Bureau of Standards. 



2 Proc. Roy. Soc, 83: 548-561. 1910. 



3 Proc. Roy. Soc, 83: 5G1-572. 1910. 



1 



