378 Prof. E. Wiedemann on the 



stances, e. g. the calcium-sulphide combinations, agrees with 

 the experimental results as to the diminution of brightness. 

 At the same time it was determined how much of the absorbed 

 energy was again radiated as light. 



Bal main's luminous paint was employed as the phos- 

 phorescent substance ; it was formed into a plate by mixing 

 the powder with aqueous gelatine, and drying the mixture on 

 glass plates provided with a paper border. In this way very 

 fine homogeneous thin films were obtained, which phos- 

 phoresced well. 



The following experimental arrangement was made. The 

 rays coming from a Schuckert's arc lamp, L (Plate III. fig. 4), 

 fell upon the slit, S, of a spectroscope ; the collimator-lens, 

 C, rendered them parallel ; then they traversed a prism, P, 

 and an achromatic lens, A, of 1 metre focal length, which 

 threw a real spectrum upon a second slit, Si (the observing- 

 telescope was turned to one side). From this spectrum the 

 slit S x admitted the strongly active portion of the ultra-violet 

 rays, which fell then upon a plate, B, of Balmain's paint 

 supported on a glass plate, G, at an angle of 45° to the direc- 

 tion of the rays. From the phosphorescent light emitted in 

 all directions a cone of rays was allowed to pass through a 

 small opening, co, in a screen, Q, which was placed parallel to 

 the rays passing through S 1? of which the rays were rendered 

 parallel by the lens, L lt The objective, 0, of a Zollner's 

 astro-photometer produced a sharp image of the small screen, if 

 this were illuminated. If the luminous paint had been ren- 

 dered phosphorescent, then there was seen in the field of view 

 of the photometer a sharp image of the screen, together with 

 the comparison-star. This was made of similar colour to the 

 first by means of a colorimeter, and was then reduced to equal 

 brightness. 



After the rest of the apparatus had been adjusted, the plate 

 of Balmain's luminous paint, which had been a long time in 

 the dark, and which showed no trace of luminosity even in a 

 perfectly dark room, was put into its place by the aid of a 

 small reading-off lamp, which caused no perceptible excite- 

 ment. The light of the electric lamp was completely shut off 

 by means of a screen placed immediately in front of the col- 

 lecting-lens. One observer then removed the screen from the 

 lamp during a definite time, as determined by the beat of a 

 seconds pendulum, and thus allowed a definite quantity of 

 light to fall upon the luminous tablet, whilst another observer 

 sat in front of the photometer with closed eyes. As soon as 

 the screen had been replaced the second observer adjusted the 

 photometer as quickly as possible, so that the comparison- star 



