PHILOSOPHICAL SOCIETY OF WASHINGTON. 281 



moon received a photographic impression, while the lime simi- 

 larly exposed, exhibited no phosphorescence.* 



Henry was afterward accustomed on the occurrence of a bright 

 aurora, to expose a sheet of paper written or figured with a 

 solution of bisulphate of quinia to the auroral light, when the 

 characters (quite invisible by lamp-light or even by day-light) 

 would distinctly glow with a pale blue light ; — indicating the 

 electrical nature of the meteor. 



In January, 1845, in conjunction with Professor Stephen Alex- 

 ander, he instituted a series of experimental observations on the 

 relative heat-radiating power of the solar spots. On the 4th of 

 January a large spot through which our terrestrial globe could 

 have been freely dropped, (having been estimated at more than 

 10,000 miles in diameter,) favorably situated near the middle of 

 the disk, was examined with a telescope of four inches aperture. 

 A screen having been arranged in a dark room, with a thermo- 

 electric apparatus behind it and having its terminal or pile just 

 projecting through a hole in the screen, the image of the spot 

 was received upon it, giving a clearly defined outline about two 

 inches long and one inch and a half wide. By a slight motion 

 of the telescope the spot could readily be thrown on or off the 

 end of the pile as desired. A considerable number of observa- 

 tions indicated very clearly by the differing deflections of the 

 galvanometer needle " that the spot emitted less heat than the 

 surrounding parts of the luminous disk."f A brief account of 

 the results obtained by these researches given in a letter to his 

 friend Sir David Brewster, was read by him at tiie Cambridge 

 Meeting of the British Association in June, 1845. | The deter- 

 minations arrived at have been fully confirmed by the later obser- 

 vations of Secchi and others § 



In 1845, he contributed a paper to the Princeton Review, on 

 "Color Blindness;" which although in the modest form of a 

 literary review of two Memoirs then recently published, (that of 

 Sir David Brewster in the Philosophical Magazine ; and that of 



* Proceed. Am. Phil. Soc. May 26, 1S43, vol. iii. pp. 3.8-44. This in- 

 teresting but oliscure sutiject althougli apparently connected witii tlie 

 phenomenon of " fluorescence" has yet an entirely distinct phase in its 

 abnormal continuance of luminosity, — similar to the familiar etfect of a 

 thermal impression. It is possible however that the conversion of wave- 

 peviodicily (wave-length), shown by Stokes to be the characteristic of 

 fluorescence, may require time for its full development. 



t Procfed. Km. Phil. Soc. June 20, 1845, vol. iv. p. 176. 



X Report Brit. Aftsnc. 1846, part ii. p. 6. 



§ P. Angelo Secchi — during the years 1848, and 1849, was Professor of 

 Mathematics at the College of Georgetown, D. C. : and in the preparation 

 of his " Researchf-s on Electrical Rheometry " published in the third 

 voluuje of the Smithsonian ContrihutioriK, (aj-t. ii. p. 60.) lie received from 

 Henry the friendly assistance of apparatus and susrgestious. He was ap- 

 pointed Director of the Observatory at Rome, iu 1850. 



55 



