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Prof. G. G. Stokes on certain 



[Dec. 20, 



to radiations in general (especially luminous radiations, which are the 

 most easily studied) with what we observe as to the motions of radio- 

 meters, we may arrive at some probable conclusions. 



17. We may evidently conceive a series of ethereal vibrations of any 

 periodic time, however great, to be incident on a homogeneous medium 

 such as glass, and inquire in what manner the rate of absorption would 

 change with the period ; though whether we can actually produce ethereal 

 vibrations of a very long period is another question, seeing that we can 

 only act on the ether by the intervention of matter, and are limited to 

 such periods of vibration as matter can assume when vibrating molecu- 

 larly,. in a manner communicable to the ether, and not as a continuous 

 mass, in the manner of the vibrations which produce sound. We may 

 inquire whether, on continually increasing the period of vibration, the 

 glass (or other medium) would ultimately become and remain very 

 opaque, or whether, after passing through a range of opacity, it would 

 become transparent again, on still further increasing the period of the 

 incident vibrations. 



18. This is a question the experimental answer to which, as it seems 

 to me, could only be given, in so far as it could be given at all, as a result 

 of a long series of experiments, of a kind that Melloni has barely touched 

 on. A variety of considerations, which I could not explain in short com- 

 pass, lead me to regard the second alternative as the more probable, 

 namely, that, on increasing the periodic time, homogeneous substances in 

 general (perhaps even metals, though this is doubtful) become at last 

 transparent, or at least comparatively so. The limit of opacity, in all pro- 

 bability, varies from one substance to another ; and the lower it is, the 

 lower would be the lowest refrangibility of the radiation which the same 

 substance is capable of emitting. 



19. In what immediately follows I shall suppose accordingly that glass 

 is strongly absorbing through a certain range of low refrangibility, on 

 both sides of which it gradually becomes transparent again*. Imagine a 

 spectrum containing radiations of all refrangibilities with which we have 

 to deal ; let portions of this spectrum on the two sides of the region of 

 powerful absorption for glass be called wings of that region, and let left to 

 right be the order of increasing refrangibility. Then the spectrum of the 

 radiation from a thin plate of glass, if it could be observed, would be 

 seen to occupy the region of chief absorbing (and therefore emitting) 

 power and its wings. The spectrum of the radiation from the outer 

 stratum of the bulb of the pith radiometer, after transmission through the 

 inner, would consist of two wings, with a blank, or nearly blank, space 

 between ; it would resemble, in fact, a widened bright spectral line, with 



* It may be noticed that this supposition, which, as appearing the more probable, 

 is adopted for clearness of conception, is not essentially involved in the explanation 

 that follows, which would hardly be changed if the "left wing" were not terminated 

 on the left. 



