496 NICHOLS— COLOR-PHOTOGRAPHS OF 



rent wave and the sectored disk was shifted until the sparking 

 occurred during eclipse, i. e., at the time when a closed sector ob- 

 scured from view both the sparks and the substance under excita- 

 tion (see Fig. 2, position i). 



With this arrangement the specimen, which was mounted verti- 

 cally in a line parallel to the row of spark gaps and distant from 

 them about 2 cm., was intensely illuminated 120 times a second and 

 was visible during intermediate intervals of 1/240 of a second, each 



Fig. 2. 



beginning about .0001 second after excitation ceased (see Fig. 2, 

 position 2). 



For observations during later stages of decay the wheel could be 

 driven slowly by means of a direct-current motor or moved stepwise 

 an eighth of a revolution at a time at the desired rate. 



Since some of the most striking changes of color are produced 

 by differences of temperature, the tube containing the phosphorescent 

 sulphide was mounted within a cylindrical Dewar flask with un- 

 silvered walls. The lower end rested in a metal mercury cup while 

 the upper end passed through a heating coil (see Fig. 3). From 

 the bottom of the mercury cup, M, a copper rod projected down- 

 wards into liquid air. The region at A could thus be maintained 

 at a temperature of + 20° C, or higher, while the lower end at B 

 was at approximately — 185° C. 



A fairly stable temperature gradient soon established itself and 

 the colors of phosphorescence through the entire range could be 

 observed and photographically recorded. 



Excitation under these conditions was obtained by inserting the 

 sparking device within the Dewar flask and many photographs were 

 made in that way; but this is a procedure demanding special pre- 



