154 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 



in the dark. We should also recollect the great sensi- 

 tiveness of the eye, and how feeble a luminous impres- 

 sion it can detect. Impressed with these facts, I have 

 endeavored to compare the absolute quantity of light 

 given by the most brilliant phosphori with some well- 

 known standards. The result of these experiments puts 

 a new view on the whole subject. 



The first attempts I made for this purpose were con- 

 ducted on the principle of comparing the stains formed 

 on a daguerreotype plate by the phosphorus under trial, 

 and by an oil-lamp, receiving the rays from each on a 

 concave metallic mirror eight inches in diameter and 

 fifteen in focus, arranged as a reflecting camera obscura. 

 There were set, side by side, a small oil-lamp, a piece of 

 white paper illuminated by the lamp, and a fragment of 

 chlorophane, arranging things in such a manner that the 

 chlorophane might be illuminated by rays coming from 

 a contact-breaker worked by two Grove's pairs. The 

 contact-breaker was kept in action fifteen minutes, and 

 then, to prove the sensitiveness of the plate, the lamp 

 was moved for one minute to a new position, and the 

 experiment closed. 



On developing, 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 maximum. 

 Also the electric spark of the contact-breaker was solar- 

 ized, and the image of the piece of white paper beauti- 

 fully given of a clear white ; but the phosphorescing 

 spar had made no impression, except from one portion 

 where it had reflected the rays of the spark. 



Suspecting that the spark from the contact-breaker 

 might not have been powerful enough, I repeated the 

 experiment, using sparks from a Leyden-jar. The oil- 

 lamp was exposed in front of the mirror one minute, and 



