166 Dr. J. W. Draper on the Phosphorogvaph 



boiling-point of mercury must be applied to a phosphorescent 

 tablet for quite a considerable time before all the light is ex- 

 tinguished. But ike red end of the spectrum and that even 

 of the diffraction-spectrum, in which the heat can with diffi- 

 culty be detected by the most sensitive thermometer, accom- 

 plishes it very quickly. 



VI. On the Infra-red Lines or Bands in the Sun's Spectrum. 



At a distance about as far below the red as the red is below 

 the yellow in the solar spectrum, I found in 1842, in photo- 

 graphs taken on iodide of silver (Daguerre's preparation), 

 three great lines or bands, with doubtful indications of a 

 fourth still further off. I designated them as a, /3, 7, and 

 published an engraving of them in the Philosophical Maga- 

 zine for May 1843. 



In 1846, MM. Foucault and Fizeau having repeated the 

 experiment thus originally made by me, presented a com- 

 munication to the French Academy of Sciences. They had 

 observed the antagonizing action above referred to, and had 

 seen the infra-spectral lines a, /3, 7. They had taken the 

 precaution to deposit with the Academy a sealed envelope, 

 containing an account of their discovery, not knowing that 

 it had been made and published long previously in America. 



Sir J. Herschel had made some investigations on the dis- 

 tribution of heat in the spectrum, using paper blackened on 

 one side and moistened with alcohol on the other. He ob- 

 tained a series of spots or patches, commencing above the 

 yellow and extending beyond the red. Some writers on this 

 subjecthave considered that these observations imply a discovery 

 of the lines a, /3, 7. They forget, however, that Herschel did 

 not use a slit, but the image of the Sun, an image which 

 was more than a quarter of an inch in diameter. Under 

 such circumstances, it was impossible that these or any other 

 of the fixed lines could be seen. 



I have many times repeated this experiment, but could not 

 obtain the same result, and therefore attributed my want of 

 success to unskilfulness. More recently Lord Rayleigh (Phil. 

 Mag., November 1877), having experimented in the same 

 direction, seems to be disposed to attribute these images to a 

 misleading action of the prism employed. Whatever their 

 cause may be, it is clear that they have nothing to do with 

 the fixed lines a, /3, 7 now under consideration. 



In these experiments, and also in others made about the same 

 time on the distribution of heat in the spectrum, I attempted 

 to form a diffraction-spectrum without the use of any dioptric 



