Prof. Draper on the Phosphorescence of Bodies. 93 



Determination of the absolute quantitij of Light emitted by 

 Phosphori. 



And now we may fairly inquire how it is with the light itself ? 

 do we not deceive ourselves respecting it ? We ought to recol- 

 lect that it is barely perceptible in the open day, and that these 

 experiments require to be made in the dark. We should also 

 recollect the great sensibility of the eye, and how feeble a lumi- 

 nous impression it can detect. Impressed with these facts, I 

 have endeavoured to compare the absolute quantity of light given 

 by the most brilliant phosphori with some well-known standards. 

 The result of those experiments, now to be presented, puts a new 

 view on the whole subject. 



The first attempts I made for this pui-pose were conducted on 

 the pi-inciple of comparing the stains formed on a DagueiTCotype 

 plate by the phosphorus under trial and by an oil-lamp, receiving 

 the rays from each on the polished concave mirror, eight inches 

 in diameter and fifteen in focus, of a reflecting camera-obscura. 

 For this purpose there were set, side by side, a small od-lamp, 

 a piece of white paper illuminated by the lamp, and a piece of 

 chlorophane, arranging things in such a way that the stone 

 might be illuminated by rays coming from a contact-breaker 

 worked by two Grove's pairs. Tlie contact-breaker was kept in 

 rapid action for fifteen minutes ; and then, to prove the sensi- 

 tiveness of the plate, the lamp was moved for one minute to a 

 new position, and the experiment closed. 



On mercurializing, it was found that the impressions of the 

 lamp had solarized, both that of fifteen minutes and that of one, 

 proving that such a light in one minute is amply sufficient to 

 change the plate to its maximmu. Also the electric spark of the 

 contact-breaker was solarized, and the image of the piece of white 

 paper beautifully given, of a clear white ; but the phosphorescing 

 spar had made no impression, except from one portion, where it 

 reflected the rays of the spark. 



Suspecting that the spark from the contact-breaker might not 

 have been jjowerful enough, I repeated the experiment, using 

 sparks from a Leydcn jar. "The oil-lamp was exposed in front of 

 the camera one minute and then removed; then ten strong 

 sparks were passed over the spar, each of which made it emit an 

 emerald light ; but during the moment of the passage of each 

 spark a screen was interposed, that no direct or reflected light, 

 and indeed none but that of the phosphorus, could reach the 

 reflecting camera and sensitive plate. 



On mercurializing, it was found, as before, that the lamp was 

 beautifully dc])icted, but the sjjar invisible. 



Estimated, therefore, by the chemical effects they can produce, 



