OF ARTS AND SCIENCES. 227 



on the phosphorus from vvhicli that copy was taken. This arises from 

 the fact that the eye is made less sensitive by the light emitted from 

 surrounding phosphorescent parts, and cannot perceive a sombre point 

 or line among them. That is a physiological effect. But a gelatine 

 copy in no respect dazzles or enfeebles the eye. For this reason, for 

 instance, we may not be able in a phosphorograph to resolve visually 

 the infra-red briglit rectangle into its constituent lines, but we recog- 

 nize them instantly in the gelatine. 



I have made use of sensitive gelatine plates ever since their quality 

 of being affected by phosphorescent light was announced by Messrs. 

 Warnecke and Darwin. The more sensitive of these plates receives a 

 full effect by an exposure of less than one minute. 



But all kinds of phosphori will not thus affect a photographic tablet : 

 there must be a sympathy between the jjhosphorescent and the photo- 

 graphic surfaces. Thus a phosphorus emitting a yellow light will not 

 affect a photographic preparation which requires blue or indigo rays. 

 This principle I detected many years ago. In my memoir on phos- 

 phorescence (Phil. Mag., February, 1851), it will be seen that the 

 green light emitted by chlorophane could not change the most sensi- 

 tive photographic preparation at that time known — the daguerreotype 

 plate — and hence I was obliged, in measuring the light it emits, to 

 resort to Bouguer's optical method. The result would have turned out 

 differently had the light to be measured been more refrangible, blue 

 or indigo or violet. 



A photographic surface agrees with the retina in this, that it has 

 limits of sensitiveness. The eye is insensible to rays of much lower 

 refrangibility than A, and much higher than 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 

 j)lace or point of maximum sensitiveness. But some substances, such 

 as the iodide and bromoiodide of silver, under special methods of 

 treatment are affected either positively or negatively throughout 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. Between 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 de- 

 clines. It is necessary, therefore, to make that interval between the 

 two moments as short as possible. 



