56 SCIENCE PROGRESS 



diamonds, which are really phosphorescent l ; and subsequently 

 Dufay showed that a fresh exposure to light would again render 

 the stone and other phosphoroids phosphorescent after their 

 power of emitting light had been destroyed by heating. The 

 first artificial phosphoroid was prepared by Peter of Bologna, 

 the "Bologna stone" (about 1602). This is barium sulphide 

 containing traces of foreign metals, which, as we shall see later, 

 are essential for the phosphorescence. With the aid of this 

 phosphoroid Zanotti, using the solar spectrum, established the 

 important fact that the colour of the emitted phosphorescent 

 light is independent of the colour of the exciting light ; Dufay, 

 using coloured glasses, established the same fact for diamonds 

 and, as already stated, recognised that, in the case of phosphores- 

 cence consequent on heating, a previous excitation was necessary. 

 Later on Wilson showed that a great number of phosphorescent 

 shells each emitted light of a fixed colour, whatever the colour 

 of the exciting light. The next fundamental observation, that 

 the red and infra-red rays extinguish a glowing phosphoroid — 

 i.e. cause the parts on which they fall to lose their luminosity 

 much faster than the other parts — was first made at the begin- 

 ning of the nineteenth century by Ritter, though the first easily 

 accessible reference is to be found in the poet Goethe's scientific 

 works (Farbenlehre, § 678). This phenomenon was rediscovered 

 by E. Becquerel, who noticed also that when infra-red light was 

 first thrown on the phosphoroid a momentary increased lumino- 

 sity was noticeable, which was followed by the rapid decay of 

 intensity just mentioned, so that the parts of a phosphorescent 

 sheet struck by infra-red radiations first become brighter than 

 the other parts but soon afterwards become much darker. He 

 observed that the effect of light was similar to that produced 

 by directly heating the phosphoroid and used these properties 

 in investigating the infra-red solar spectrum. He also made 

 an extensive series of observations on the spectra of phos- 

 phorescent substances by throwing a spectrum on to plates 



1 It was generally thought by the ancients and in mediaeval times — Pliny, 

 Solinus, Isidor of Seville— that the ruby and carbuncle shone in the dark ; though 

 no phosphorescence of any duration is obvious in the case of these stones, the 

 ruby shows the phosphorescence of very short duration usually called fluorescence ; 

 in fact, the genuineness of the stone may be tested by exposing it to blue light, 

 when the true ruby— which may, however, be synthetic— emits red light ; a paste 

 imitation only reflects the blue. 



