528 M. Becqucrel on the Luminous Effects 



In a fourth chapter, M. Becquerel treats of the intensity and 

 composition of the light emitted by phosphorescent bodies at 

 different temperatures. On this subject he remarks that, when 

 diffuse or solar light is used to excite the substances, the light 

 emitted by phosphorescence is much less brilliant than the in- 

 cident light ; but if different parts of the spectrum are operated 

 with, this is not always the case ; for, as already observed, the 

 invisible rays beyond the violet give rise to a brilliant phospho- 

 rescence. The phosphoroscope, by enabling the author to follow 

 continuously the luminous emission of a body, furnished him 

 with the means of studying the changes which take place in the 

 intensity of the phosphorescent light when the intensity of the 

 active rays varies within certain determinate limits. He found, 

 in general, that the intensity of the light emitted by a body in 

 virtue of its proper action, whilst remaining independent of the 

 duration of insolation, is a function of the intensity of the 

 exciting rays. With respect to the composition of the light 

 emitted by a phosphorescent bod)', M. Becquerel found that 

 in general it had a refrangibility less than that of the exci- 

 ting rays ; but in this case, as in that of the emission of rays 

 by any luminous source whatever, the colour of the emitted 

 light is intimately connected with the refrangibility of the 

 exciting rays. It further appears that the intensity and the 

 composition of the light emitted by phosphorescent bodies de- 

 pend as much upon their temperature as upon their physical 

 constitution. In fact the author has shown that at an elevated 

 temperature a body becomes capable of emitting rays of a cer- 

 tain tint whose duration and intensity are both determinate ; and 

 that, on cooling to its ordinary temperature, the body in question 

 is subject to the same conditions as at first. The substance 

 which in this respect presents the most remarkable modifications 

 is sulphuret of strontium, obtained by the reaction of sulphur 

 upon strontia at a temperature above 500° or 600°. In this 

 case the emitted light, which is of a violet colour at the ordinary 

 temperature, becomes clear bhie at 40°, greenish at 70°, green- 

 ish yellow at 100°, and at length orange at about 200°, thus 

 becoming more and more refrangible as the temperature increases. 

 The author has collected in a Table a series of observations made 

 on the differences between the tints of the light emitted by the 

 principal types of phosphorescent sulphurets raised to various 

 temperatures between —20^ and -{-250°. 



The intensity and the duration of phosphorescence, as well as 

 the refrangibility of the emitted light, vary when the bodies are 

 retained at different temperatures, but they do so unequally for 

 each substance. The author has proved that in general almost 

 all phosphorescent substances receive a greater totality of action 



