94 Prof. Draper on the Phosphorescence of Bodies. 



the liglit from chloropliane is incomparably less intense than 

 that from a common lamp. For there can be no doubt that 

 each of the ten discharges gave a light which lasted brilliantly 

 for six seconds, and upon the whole was equal in duration to the 

 light of the lamp ; yet the latter had changed the plate to a 

 maximum, while the former had not made the smallest percep- 

 tible impression. 



As the foregoing attempts to obtain photographic effects had 

 failed, I next varied the experiment as follows : — In a Bohemian 

 glass tube a quantity of chlorophane was placed, in coarse frag- 

 ments, sufficient to occupy about three inches in length of the 

 tube. The reflecting camera was placed in a proper position, 

 accommodated with its sensitive silver plate. Allien everything 

 was arranged, a spirit-lamp was applied to the chlorophane, 

 which soon emitted a superb emerald light, and continued to do 

 so for about two minutes. As the light began to decline, the 

 spar splintered by decrepitation, but all the process went fonvard 

 in a very satisfactory way. 



As soon as this was complete, I placed an oil-lamp in front of 

 the camera for live seconds. 



On mercurializing, the picture of the lamp-flame came out, 

 but no trace whatever of the chlorophane could be detected. 



So it appears that the splendid green light emitted when the 

 spar is heated, is at least tv\'enty-four times less intense than the 

 light emitted by a small oil flame. It should be remembered 

 that this is a measure of absolute intensity, and not of illumi- 

 nating power. 



But as it is known that green rays are not veiy efficient in 

 changing a sensitive surl'ace, I concluded to detennine the inten- 

 sity of light emitted by chlorophane by the optical method of 

 Bouguer, a description of which is given in my paper on the 

 Production of Light by Heat, in the Philosophical Magazine for 

 May 1847. 



The spar being heated by a current of hot air arising from the 

 flame of a spirit-lamp, the light of which was carefully screened 

 by a chimney, and other contrivances of sheet iron, a comparison 

 was made with a veiy small oil-lamp, the flame of which was 

 about six-tenths of an inch high, and the wick one-sixth of an 

 inch thick, covered with a glass shade. 



On making the experiment, eveiything went on veiy well. The 

 spar, when it began to glow, cast a reddish shadow on the paper, 

 which shadow was extinguished when at its maximum by the 

 lamp at about 25 inches, the spar being at 5 inches. 



The distances of the chlorophane and lamp from the paper 

 were therefore as 1 : 5. The illuminating effect is as the squares 

 of those numbers, and therefore 1 : 25. But for extinction, it 



