1G2 Dr. J. W. Draper on the Phosphoroc/raph 



H. Gelatine cannot perceive rays lower than F ; but it is 

 affected by others far higher than H. There is therefore a 

 range for each, having its limits and also its place or point of 

 maximum sensitiveness. But some substances, such as the 

 iodide and bromoiodide of silver, under special methods of 

 treatment, are either affected positively or negatively through- 

 out the entire range of the spectrum. 



In experiments for obtaining quantitative results, it should 

 be borne in mind that there is generally a loss of effect. Be- 

 tween the moment of insolation and that of perception, either 

 by the eye or by gelatine, emitted light escapes. The moment 

 of maximum emission is the moment of completed insolation ; 

 and from this the light rapidly declines. It is necessary, 

 therefore, to make that interval between the two moments as 

 short as possible. 



V. Of the Extinction of Phosphorescence by Red Light. 



I turn now to an examination of those parts of the phos- 

 phorographic spectrum from which the light has been removed. 

 They are from the line F to the end of the infra-red space, 

 and again for a short distance above the violet. The effect 

 resembles the protecting action in the same region of a 

 photograph. 



.Now, if similar effects are to be attributed to similar causes, 

 we should expect to find in the photograph and phosphorograph 

 the manifestation of a common action. 



Several different explanations of the facts have been offered. 

 Herschel suggested that the photograph might be interpreted 

 on the optical principle of the colours of thin films. Very 

 recently Captain Abney has attributed the appearance of the 

 lower space to oxidation. But this can scarcely be the case 

 in all instances. Mr. Claudet showed, in a very interesting 

 paper on the action of red light, that a daguerreotype plate 

 can be used again and again by the aid of a red glass, and 

 that the sensitive film undergoes no chemical change (Phil. 

 Mag., February 1848). 



It was known to the earliest experimenters on the subject 

 that if the temperature of a phosphorescent surface be raised, 

 the liberation of its light is hastened, and it more quickly 

 relapses into the dark condition. In the memoir to which I 

 have previously referred (Phil. Mag., February 1851), I 

 examined minutely into this effect of heat, and determined 

 the conditions which regulate it. And since, on the old view 

 of the constitution of the solar spectrum, the heat was supposed 



