938 



SCIENCE 



[N. S. Vol. XLIII. No. 1122 



vertical axis, A, Fig. 2, and carrying a layer 



of the phosphorescent substance on its periph- 

 ery. 



A source of light, S, illuminates the travel- 

 ing surface through a fixed vertical slit and the 

 various stages of the phosphorescent glow may 

 be studied by observing at different angles 

 from this opening. In the form used by 

 Waggoner, where a wheel of 45 cm. diameter 

 was used instead of the small drum of Tyndall 

 and of Kester this instrument has many ad- 

 vantages over other forms provided the sub- 

 stance is available in sufficient quantity to coat 

 the entire rim of the wheel. With a speed suit- 





©■ 



■e- 



Fig. 3. 



able to the substance under observation the 

 whole phenomenon of the decay of phosphores- 

 cence may be viewed at a glance, including 

 progressive changes of intensity and color. 



The instrument, which we have called the 

 synchronophosphoroscope and which is to be 

 described in the present note, was devised for 



various studies in phosphorescence to which 

 the previous tj^ies are not easily adaptable. It 

 consists of a small synchronous, alternating- 

 current motor A. C, Fig. 3, and a small di- 

 rect-current motor D. C. upon a common shaft. 

 To one end of the shaft is attached a sectored 

 disk, WW, Figs. 3 and 4, with four equal open 

 and four closed sectors, corresponding to the 

 four poles of the A. C. motor. On a circuit of 

 60 cycles this machine, when brought to speed 

 by the D. C. motor and released, runs steadily 

 at 30 revolutions per second. A " step up " 

 transformer TT, in the same alternating cur- 

 rent circuit produces discharges at the spark 

 gap, or series of gaps (E), at each alternation, 

 i. e., 120 times a second. This discharge may 

 be reduced to a single spark by proper adjust- 

 ment of the resistance and capacity of the cir- 

 cuit or more conveniently for many purposes 

 the discharge may be confined to the peak of 

 the wave by means of the four-pointed star- 

 wheel, SS (Figs. 3 and 4), which is mounted 



® 



FiQ. 4. 



on the shaft and carefully adjusted as to 

 phase. 



The direct-current motor may also be used 

 to drive the sectored disk at other speeds, in 

 which case the circuit of the motor A. C. is 

 broken and the discharge is derived from any 

 convenient source capable of producing a 

 proper spark at each quarter revolution. 



When the sectored disk WW is so adjusted 

 on the shaft that the closed sectors conceal the 

 phosphorescent surface during excitation by 

 the spark, an observer, looking through the 

 open sectors as they pass, sees the phosphores- 

 cence as it appears a few ten thousandths of a 

 second after. The apparatus is thus suitable 

 for the study of phosphorescence of very short 

 duration or of the earliest stages in cases of 

 slow decay. By shifting the sector on the 



