PHILOSOPHICAL SOCIETY Of WASHINGTON. 135 



years ago, to find our college professors zealously inculcating the 

 undulatory theory of light, while still maintaining the hypothesis 

 of a "caloric " for heat. 



William Hersche.l had found, at the beginning of the century, 

 that the solar spectrum, as produced by an ordinary glass prism, 

 manifested a heating power slight at the violet end, but gradually 

 increasing to the red end, and extending a considerable distance 

 beyond the less refrangible limit of visible rays, near which limit 

 the maximum effect was reached.* 



Johann Wilhelm Ritter, of Jena, a year later found that the 

 chemical action of the solar spectrum, as exhibited in the darkening 

 of silver chloride, increased toward the violet extremity, attaining 

 a maximum beyond the most refrangible limit of luminous disper- 

 sion.f Hence, it came to be generally believed that the solar rays 

 comprise three essentially distinct and independent kinds of energy, 

 representing three different forms of wave-motion. This appeared 

 the more probable from the entirely dissimilar orders of effect 

 observed (as interpreted by the impressions of our senses), in calor- 

 ific energy, in optical luminosity, and in chemical agency. 



It was shown however by Alexandre Edrnond Becquerel that 

 the so-called chemical rays were not distinguishable by their re- 

 fraugibility, and that photographic effects could be obtained with 

 suitable re-agents from any region of the spectrum. J And finally, 

 by the researches of Dr. John W. Draper, it was fully established 

 that Herschel's results depended on the great distortion (as well as 

 unequal absorption) inseparable from every prismatic or refractive 

 spectrum, and that Ritter's results depended on a very limited and 

 insufficient induction. And thus it has slowly come be recognized 

 that in every normal spectrum, freed from distortion or selective 

 absorption, (and equally freed from selective generalization), the 

 three classes of effects, thermal, photic, and actinic, are equably 

 or proportionally distributed; that as these several activities are 

 equally amenable to polarization, to interference, and to spectral 

 irradiation and absorption, there is in fact but a single form of 



*Phil. Trans. Roy. Soc. 1800: vol. xc, pp. 291, 318, 439, 440. 



f Gilbert's Annalen der Physik. 1801: vol. vn, p. 527. Nicholson's 

 Journal of Natural Philosophy, [etc.] August, 1803 : vol. v, p. 255. 



\ Annates de Chemie et de Physique. April, 1849: vol. xxv, pp. 447- 

 474. 



