OP ARTS AND SCIENCES. :^81 



exposure of three or four seconds was sufficient to give a fair i)roof. 

 But, on insolating a phosphorescent tablet, and causing the converging 

 moon rays to pass through the red glass which I commonly use as 

 an- extinguisher, no effect was produced by the red moonlight on the 

 shining surface. 



I repeated this experiment using a lens 5 inches in diameter and 7 

 inches focus so arranged that the moon's image could be kept station- 

 ary on the phosphorescent tablet. That image was about ^ inch in 

 diameter. Then, insolating the tablet, the moon rays, after passing 

 through a red glass, were caused to fall upon it. The exposure con- 

 tinued ten minutes, but no effect was produced on the shining surface. 

 The lunar image was so brilliant that when the red glass was removed, 

 and a non-shining phosphorescent surface was exposed to it, a bright 

 image could be produced in a single second. 



But in order to remove the effect of the more refrangible rays by 

 the less, the latter must not only have the proper wave length but also 

 the proper amplitude of vibration. This principle applies both to 

 photographic and phosphorographic experiments. In my memoir on 

 the negative or protecting rays of the sun (Phil. Mag., February, 

 1847) it is said, " Before a perfect neutralization of action between 

 two rays ensues, those rays must be adjusted in intensity to each other." 

 It requires a powerful yellow ray to antagonize a feeble daylight. 



It is owing to the difference in amplitude of vibration that the heat 

 of radiation seems so much more effective than the heat of conduction. 

 A temperature answering to that of the boiling point of mercury must 

 be applied to a phosphorescent tablet for quite a considerable time 

 before all the light is extinguished. But the red end of the spectrum 

 and that even of the diffraction spectrum, in which the heat can with 

 difficulty be detected by the most sensitive thermometer, accomplishes 

 it very quickly. 



VI. Op the Intba-bbd 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 photographs 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, y, and published an engraving of them in the Philosophi- 

 cal Magazine for May, 1843. 



In 1846, MM. Foucault and Fizeau having repeated the experi- 



