JOHN WILLIAM DRAPER. 



with and married in heaven. The fates had made him immortal ; 

 but, unlike his bride, in the course of events he became feeble and 

 decrepit, and, losing all his strength, was rocked to sleep in a cradle. 

 The goddess, pitying his condition, metamorphosed him into a grass- 

 hopper. The fact and the fable agree pretty well, and, indeed, the 

 playful coincidence might be carried much farther. The powers of 

 photography, which bring architectural remains and the forms of 

 statuary so beautifully and impressively before us, might seem to be 

 prefigured by the speaking image of the son of Tithonus and Au- 

 rora that was to be seen in the deserts of Egypt. Besides this such 

 words as tithonoscope, tithonometer, tithonography, tithonic effect, 

 ditithonescence are musical in an English ear." In the same year 

 he described a tithonometer or instrument for measuring these chem- 

 ical rays based on their action upon chlorine. The apparatus con- 

 sisted of an inverted siphon tube, the shorter limb of which was 

 closed, the longer drawn out and graduated. By means of wires of 

 platinum sealed into the shorter limb the solution of hydrochloric 

 acid which it contained could be decomposed by a voltaic current, 

 and this limb filled with mixed hydrogen and chlorine gases. When 

 the image of a flame, formed by a convex lens, was caused to fall 

 on the sentient tube the liquid in the longer limb began instantly to 

 descend, moving regularly over the scale so long as the exposure was 

 continued. This instrument, while much simpler, appears to be 

 quite as sensitive as the one described by Bunsen and Koscoe many 

 years later (1856). 



The third method which Dr. Draper employed for studying the 

 action of the chemical rays was based on the growth of plants. 

 Already, in 1837, he had investigated the phenomenon of the de- 

 composition of solar light by leaves, and had shown that these leaves 

 absorbed certain rays. But these early experiments were made 

 under colored glasses and were not entirely conclusive, other rays 

 passing simultaneously through the media used. When seeds were 

 made to germinate under these glasses, however, he found that the 

 plants under the red and the violet glasses were as perfectly etiolated 

 as if they had been kept in the dark ; while those under the yellow 

 glass promptly assumed a green color and developed rapidly. In 

 order to obviate the objection raised to colored glasses a crop of 

 seeds was caused to germinate in a long box placed in the dark and 

 the young plants were then exposed to the action of a solar spectrum. 

 Those in the yellow speedily turned green, while those in the red 

 (2) 365 



