NATIONAL ACADEMY OF SCIENCES. 



the apparent brightness of the yellow, as seen by the eye, is a purely 

 physiological phenomenon. 



Another subject to which Dr. Draper devoted a large share of 

 attention was phosphorescence. In his first memoir on this subject, 

 published in 1851, he discusses the general character of the phenom- 

 enon, and restricts the term phosphorescent to those bodies which 

 shine in the dark after exposure to light or on being heated. Fluor- 

 spar, of the variety known as chlorophane, which yields a superb 

 emerald-green light, was selected for experiment, He found that 

 when made to phosphoresce by the electric spark it underwent neither 

 expansion nor contraction, nor any other molecular change detectable 

 in polarized light. He did observe a minute evolution of heat, but 

 could detect no electrical change. Nor did the presence of a pow- 

 erful magnetic field appear to affect the result. On measuring the 

 amount of light emitted it appeared that this splendid green light 

 was photographically twenty- four times, and photometrically three 

 thousand times, less intense than that of the small flame of an oil 

 lamp taken for comparison. But he noticed that the quantity of 

 light emitted by a phosphorescent body was proportional to the in- 

 tensity of the light to which it had been exposed. In 1844 Dr. 

 Draper determined that the special phosphorogenic rays of the 

 spectrum were the violet rays. In this research he used a quartz 

 train and threw the spectrum upon a screen covered with calcium 

 sulphide. He observed, further, that for the transient light of the 

 spark quartz is transparent but glass is opaque that is, so far as a 

 surface of calcium sulphide is concerned, though not a sensitive 

 silver surface. Moreover, while glass is opaque to phosphorogenic 

 rays from the spark, it transmits freely those from incandescent lime, 

 so that the calcium light, the light of an oil lamp, and sunlight can 

 excite phosphorescence through glass ; that of the electric spark or 

 of the voltaic discharge in mercury cannot. If, however, the latter 

 be continuous an effect is produced even through glass. 



In his last scientific memoir, published near the close of 1880, 

 Dr. Draper called attention to the striking resemblance existing 

 between a photograph of the solar spectrum taken on silver iodide 

 and a phosphorograph taken on luminous paint. The former, when 

 taken in presence of a weak extraneous light, shows the three 

 regions which were pointed out by him in 1842 : (1 ), a middle region, 

 extending from the boundary of the green and blue to a little be- 

 yond the violet ; here the silver iodide is blackened ; (2), below this 



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