Vol. 7, 1921 PHYSICS: LOEB AND SCHMIEDESKA MP 
203 
different samples of phosphorescent zinc sulfides were used. One was a 
sample obtained from the Radium Standard Chemical Company in 1918, 
the others were two samples obtained in France from MM. Guntz and 
Ziegler of the powder works at Bouchet in 1918. The specimens to be 
studied were prepared by sifting the fine crystals upon small circular 
plane glass plates, 7 mm. in diameter which had been moistened with 
a very dilute solution of collodion in amyl acetate (about 3 drops of an 
ether collodion solution of the consistency of "New Skin" in 100 cc. of 
amyl acetate). Microscopic examination of the specimens showed that 
they were covered with a uniform layer of the sulfide occupying about 
99% of the surface. The glass plate was fastened to a plate of thin trans- 
parent quartz, with the layer of sulfide pressed firmly against the quartz. 
The back side of the glass plate was then painted with optical black to 
permit the photometry of the sample. At the same time that the speci- 
men was mounted on the quartz plate a control specimen as nearly the 
same as possible was similarly mounted on a glass microscope slide. 
The quartz-mounted specimen was then placed in the image of a quartz 
Hareus mercury arc thrown upon the wall by a quartz lens of 3 cm. aper- 
ture and 15 cm. focal length. The arc operated continuously at about 
2.5 amperes in a light-tight box. A blackened thermometer bulb placed 
in the image of the arc showed a rise in temperature of but 0.5° C. above 
that of the room, so that the heating of the specimen produced by the arc 
was not of importance. The glass-mounted control sample was kept in 
a light-tight box and was removed from this only at the time of photo- 
metric measurement. 
The phosphorescent intensities of the quartz-mounted specimen and 
the control were compared by photometric measurements on the phos- 
phorescence exhibited by them when studied in a simple phosphoroscope. 
The specimens were mounted on the window of a small photometer with 
the blackened side towards the window. This window was diffusely 
illuminated from behind by a small automobile headlight lamp. The 
intensity of illumination of the window was varied, and hence the photom- 
etry was accomplished, by changing the distance of the lamp from the 
window until the phosphorescent specimen faded against the window 
which served as its background. A color match between the lamp and 
the specimen was obtained by the use of absorbing screens of colored 
glass. In the photometry the specimens were excited to phosphorescence 
by the light from a 75-watt lamp focussed on them. The exciting light 
was put on and cut off in the usual way between the intervals of observa- 
tion by means of slits in a metal disc rotated at a high speed, the procedure 
being in all respects the standard procedure used in phosphorescent 
measurements. After the quartz sample had been exposed to ultra violet 
light for a suitable interval of time, its intensity was compared with the 
