Mechanics of Luminosity. 397 



The following experiments have for their object in the first 

 place only to establish the order of magnitude of the quantity 

 b. I shall communicate further results in the continuation 

 of the investigation on phosphorescence and fluorescence. 



The divergent rays issuing from the phosphorescent body, 

 which is covered by a screen perforated by a small aperture a 

 (fig. 6), are rendered parallel by the lens L ; they fall then 

 upon the prism P, and are united by the objective of a 

 Zollner's astrophotometer to a real spectrum. 



A plate of uranium nitrate 0*1 millim. thick was employed 

 for the experiments, which was illuminated by the rays from 

 a Schuckert's arc lamp concentrated by a lens. 



A determination of the specific gravity of the uranium 

 nitrate employed in benzol gave the number 2*5. 



From the experiments with the phosphoroscope, with 

 different velocities of rotation, for which 8 lay between 0*0004 

 and 0*008 second, we obtain by the use of the above formula 



5 = 2xl0 3 , 



a number which agrees as to order of magnitude with the 

 value found by Becquerel. 



The spectrum of the phosphorescent light was found, in 

 agreement with the statements of other investigators, to con- 

 sist of a series of bright bands divided by dark spaces. The 

 comparison-star was coloured green, and the brightness was 

 determined for the three green bands in the phosphorescent 

 light when the disk of the phosphoroscope made 124 revolu- 

 tions per second. The readings a on the Nicol of the Zollner's 

 photometer, when the brightnesses are proportional to sin 2 u, 

 were: — for the green band a! = 19 c , for the yellowish-green 

 * 2 = 16°, and for the blue-green a 3 = 14°. The red and blue 

 were only weak, and too different in colour from the com- 

 parison-star to render the comparison possible. 



Then the uranium nitrate was removed, and placed before 

 the small diaphragm of the amylacetate lamp. The photo- 

 meter was read at 45° for the point in question in the yellowish 



exciting light in such directions that it does not enter the eye (or photo- 

 meter), so rendering the determination of the "brightness of the usually 

 very feeble phosphorescent light more difficult ; this is not possible with 

 small crystals or powders. On the other hand, two determinations of 

 brightness at two different velocities of rotation formed, as we have 

 shown, an excellent means of finding the brightness sought, at least very 

 nearly. The predominance of the diffusely reflected light is in fact the 

 reason why many bodies shine very brightly in the phosphoroscope, 

 whereas otherwise they require very careful examination to detect an 

 emission of light. 



