2 dorsey: luminescence measurements 



small areas of various sizes and shapes. Also, it is desirable 

 that the apparatus employed be simple in construction and use, 

 and that the results obtained be readily interpretable in terms 

 of photometric standards. 



As the luminosities frequently are low, photometers of the 

 usual types would be unsuited to the work even if their use 

 did not necessitate a determination of the effective size of the 

 preparation under study. With preparations of sufficient bril- 

 liance the spectrophotometer will give the most detailed infor- 

 mation; but this instrument is not suitable for the measurement 

 of the total luminosity or for the comparison of such lumi- 

 nosities when they differ in color. 



Other apparatus or methods that have come to our attention — 

 such as those employed by Wood, 4 Nichols and Merritt, 6 Zeller, 6 

 and Andrews 7 — all appear to be in some measure ill-adapted to 

 the present problem. The apparatus described by Andrews is 

 the simplest, but we believe that the interpretation of its read- 

 ings in terms of photometric standards will be difficult. 



In preparation for the study of self-luminous materials at the 

 Bureau of Standards, a photometer has been devised which is 

 exceedingly simple in construction and use, and appears to be 

 well suited to much of this work. By the use of standardized 

 lamps and screens the results obtained by its use can be inter- 

 preted readily in terms of photometric standards. 



In its simplest form, this photometer consists of a box 2 or 



3 meters long and 25 or 30 cm. square. A light carriage carry- 

 ing a 1.5 or 2-candle electric lamp runs on ways extending the 

 entire length of the box, the filament being in the center of 

 the section of the box. By means of two cords attached to the 

 carriage, one passing over a pulley at the rear end of the box, 

 the distance of the carriage from the end of the box can be ad- 

 justed as desired. The position of the lamp is determined by a 

 graduated steel tape having one end attached to the carriage. 



* Wood, R. W., Phil. Mag. (6) 21: 209-313. 1911. 



5 Nichols, E. L., and Merritt. E., Phys. Rev. 23:37-54. 1906; 32:38-53. 

 1911. 



6 Zeller, C. A., Phys. Rev. 31: 367-375. 1910. 



7 Andrews, W. S., Gen. Elec. Rev., 19:892-893. 1916. 



