of a Solar Spectrum. 161 



affected by the intensity of the emitted light, the gelatine by 

 its quantity. Each moment adds to the effect of the pre- 

 ceding. The gelatine absorbs all the light that the phos- 

 phorus emits from the moment of excitation, or by suitable 

 arrangement any fractional part thereof. It has another most 

 important advantage. The phosphorus is yielding an ephe- 

 meral result, and is momentarily hastening to extinction ; 

 so that for a comparison of such a result with others of a like 

 kind the memory must be trusted to. But the gelatine seizes 

 it at any predetermined instant, and keeps it for ever. These 

 permanent representations can at any future time be delibe- 

 rately compared with one another. 



To these still another advantage may be added. Very 

 frequently an impression is much more perceptible on a 

 gelatine copy than it is on the phosphorus from which 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 amongst them. This 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 phosphoro- 

 graph to resolve visually the infra-red bright rectangle into 

 its constituent lines, but we recognize them instantly in the 

 gelatine. 



I have made use of sensitive gelatine plates ever since their 

 quality of being affected by phosphorescent light was an- 

 nounced by Messrs. Warnecke and Darwin. The more 

 sensitive of the plates receives a full effect by an exposure of 

 less than one minute. 



But all kinds of phosphori will not thus affect a photo- 

 graphic tablet : there must be a sympathy between the phos- 

 phorescent and the photographic surfaces. Thus a phos- 

 phorus 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 phosphores- 

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

 green light emitted by chlorophane could not change the most 

 sensitive photographic preparation at that time known (the 

 daguerreotype plate) ; and hence T was obliged, in measuring 

 the light it emits, to resort to Bouguer's optical method. 

 The result would have turned out differentlv 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 



