ON PHOSPHOROSCOPES. 233 



I ight, always a menace in phosphorescent studies where photo-excitation is 

 used, was present, and Wiedemann appreciated the necessity of devising some 

 addition to his apparatus for the purpose of eliminating it. He favored the 

 addition of another revolving sector, similar to and coaxial with the first, 

 having openings in phase with it, and mounted between the first sector and the 

 lens. When it is considered that the intensity of the undispersed phosphor- 

 escence is from 1/100 to 1/1,000,000 that of the exciting light, it is evident 

 that the introduction of a small per cent of the exciting light into the field of 

 the phosphorescence completely aborts any attempt at quantitative measure- 

 ments. 



I 



1 

 \ / 



M 



I 



FIG. 3. 



E. Merritt 1 devised a phosphoroscope of the first type in which the phos- 

 phorescent surface P (see fig. 3) was illuminated periodically by the passage 

 of light from a spark S through an opening through a revolving disk 00'. 

 The phosphorescence could be observed at the desired later time by changing 

 the phase, while the machine is turning, of the mirror M relative to the 

 opening 0. This was accomplished in a unique manner by means of the rod 

 L, which engages a hollow sleeve R having a spiral slot cut in it. A pin 

 capable of sliding in this slot and attached to the opposite end of the same 

 inner shaft as the mirror M, is moved into different phases with the disk 00', 

 which is mounted on the outside shaft. This outer shaft is driven by a belt 

 and pulley and is keyed to the sleeve R; thus the drive is complete. Rod L 

 does not rotate, but can be locked at any desired phase. Reflection of the 

 phosphorescence, then, occurs at M , a simple photometer being employed to 

 view the light. On the revolving shaft was mounted a worm-gear for record- 

 ing the number of revolutions per minute. With this form of phosphoroscope 

 the curves of decay of many substances were traced from zero time up to 

 0.06 second. 



In the preceding phosphoroscopes the source of light has not been inter- 

 mittent, but the periodic interruption of the beam of exciting light has pro- 

 duced the effect of intermittent excitation on the specimen. Another group 

 of instruments belonging to this type employs an intermittent discharge from 

 condenser, induction coil, or transformer, as in the spark phosphoroscope of 



1 E. Merritt, Nichols and Merritt, Studies in Luminescence, Carnegie Inst. Wash. Pub. No. 

 152, p. 109. 1912. 



