226 PROCEEDINGS OP THE AMERICAN ACADEMY 



IV. Examination of Phosphorescent Tablets by 

 Gelatine Photography. 



The examination of a phosphorescent surface can be made now in 

 a much more satisfactory manner than formerly. The Hght we have to 

 deal with, being variable, declines from the moment of excitation to 

 the moment of observation. And, though the phosphori now prepared 

 are much more sensitive and persistent than those formerly made, 

 they must still be looked upon as ephemeral. To examine them 

 properly, the eye must have been a long time in darkness to acquire 

 full sensitiveness. 



It was recommended by Dufay to place a bandage over one eye 

 that its sensitiveness might not be disturbed, whilst the other being 

 left naked could be used in making the necessary preparations. But 

 this on trial will be found, though occasionally useful, on the whole an 

 uncomfortable and unsatisfactory method. 



The exceedingly sensitive gelatine plates now obtainable remove 

 these difficulties. The light emitted by blue phosphori, such as 

 luminous paint, consists largely of rays between H and G, and these 

 are rays which act at a maximum on the gelatine preparation. So if 

 a gelatine plate be laid on a shining blue phosphorus it is powerfully 

 affected, and any mark or image that may have been impressed on the 

 phosphorus will on development in any of the usual ways be found on 

 the gelatine. The gelatine has no need to wait after the manner of 

 the eye. It sees the phosphorus instantly. It is impressed from the 

 very first moment, and whilst the eye is accommodating itself and so 

 losing the best of the effect, the gelatine is gathering every ray and 

 losing nothing. Moreover, the effect upon it is cumulative. The eye 

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

 quantity. Each moment adds to the effect of the preceding. The gela- 

 tine absorbs all the light that the phosphorus 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 ephemeral result, and is momentarily hastening to extinction, so 

 tliat for a comparison of such a result with otliers of a like kind the 

 memory must be trusted to. But the gelatine seizes it at any pre- 

 determined instant and keeps it forever. These permanent represen- 

 tations can at any future time be deliberately 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 



