of a Solar Spectrum. 165 



tram formed by a reflecting grating having 17,296 lines to 

 the inch, and was not a little surprised to see that from the 

 blue to the red end of the spectrum there is an energetic ex- 

 tinction of the light, and darkness is produced. I repeated 

 this with other gratings, and under varied circumstances, 

 and always found the same effect, 



Now, considering the exceedingly small amount of heat 

 available in this case, and considering the intensity of the 

 effect, is there not herein an indication that we must 

 attribute this result to some other than a calorific cause ? 



1 endeavoured to obtain better information on this point by 

 using the rays of the moon, which, as is well known, are very 

 deficient in heating-power. Many years ago I had obtained 

 some phosphorographs of that object. With the more 

 sensitive preparations now acessible, and with a telescope of 

 11 inches aperture and 150 inches focus, there was no difficulty 

 in procuring specimens about 1*4 inch in diameter. These 

 represented the lunar surface satisfactorily. At half-moon an 

 exposure of three or four seconds was sufficient to give a fair 

 proof. 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 pro- 

 duced 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 stationary on the phosphorescent tablet. That image 

 was about i- inch in diameter. Then, insolating the tablet, the 

 moon-rays, after passing through a red glass, were caused to 

 fall upon it. The exposure continued 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 day-light. 



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 



