356 History of Luminescence 



After Becquerel, a number of phosphoroscopes "^ using new prin- 

 ciples were invented, one by Crookes (1887) for observation of 

 cathode ray phosphorescence, another by Lenard (1892) for elec- 

 tric spark ^"^ excitation, and one by Levinson (1898) for excitation 

 by scratching, a tribophosphoroscope. The Crookes and Lenard 

 instruments contained a commutator to set off the cathode discharge 

 or the electric spark at the proper instant before the luminescent 

 material could be observed. Perhaps the simplest type consists of a 

 revolving cylinder on whose surface the phosphor is cemented. If 

 illuminated with a sharp beam of light, all stages of decay of the 

 phosphorescent light can be observed at various distances behind 

 the illuminated spot as the cylinder rotates. This simple device was 

 first used by E. Becquerel (1861) , and was adopted by John Tyndall 

 for demonstration in his famous lectures on light. It was employed 

 for minerals by W. G. Levinson (1898) , and for quantitative investi- 

 gations by F. E. Kester (1899) . 



INTENSITY OF PHOSPHORESCENCE AND THE LAW OF DECAY 



In a general way the earliest observers noted that the brighter 

 the light, the more intense the phosphorescence. Probably Beccari 

 (1745) was the first to consider a relation between the two. He 

 found that one candle at a certain distance would excite the same 

 glow as four candles twice as far away. This result merely demon- 

 strates the inverse square law and shows that lights of equal inten- 

 sity excite the same luminescence. Becquerel (1861) was the first 

 to measure the excited light with a polarization photometer, find- 

 ing that within certain limits there was a direct proportionality be- 

 tween intensity of exciting and that of the emitted light. He used 

 sunlight for excitation and was surprised to find that the phospho- 

 rescent light of a uranium phosphate phosphor was only 1.5 mil- 

 lionth the intensity of the sunlight. Uranium salts were particularly 

 bright but there was much variation in other substances. The 

 double phosphate of uranium and calcium was the brightest studied 

 by him, and a wollastonite (a Ca silicate) the dimmest. 



Becquerel was the first to establish a law of decay {Abklingen in 

 German) of the emitted light, finding that the intensity decreases 

 exponentially. Later workers like L. Darwin (1881), E. Wiede- 

 mann (1888, 1889), H. Becquerel (1891, 1892), C. Henry (1892, 



^* Wiedemann (1888) modified the Becquerel phosphoroscope for high rates of rota- 

 tion and easy inspection. The various types were reviewed by Nichols and Howes 

 (Science 43: 937-939, 1916) . 



■"The "phosphoroscope ^lectrique " of Labarde {Com. Rend. Acad. Sci. 68: 1576, 

 1859) was merely a Becquerel type using electric discharges. No mention of a com- 

 mutator to trigger the sparks is made. 



