Evening Glow and analogous Phenomena. 283 



fractive action of small particles, that it is not clear what becomes 

 of the complementary blue light ; for if in other phenomena of 

 interference colours be produced from white light, this is so 

 effected that all the rays together (transmitted and reflected) 

 give white again ; in this case the light retained (reflected, ab- 

 sorbed) by the small screen is obviously white, and this should 

 be the case with the whole of the transmitted light. 



To this the following reply may be made : — The entire theory 

 developed in the preceding is based on the hypothesis that the 

 laws of diffraction are quite independent of the magnitude of the 

 apertures, however small or great they be. I do not think that 

 this assumption can be successfully contested ; it has hitherto 

 been always tacitly assumed; and I am acquainted with no fact 

 that militates against it. But, admitting that the laws of dif- 

 fraction hold for apertures however small, the principle that 

 violet rays are more enfeebled than red ones is to be considered a 

 necessary consequence of the undulatory theory. 



It is true that in other phenomena of interference, for instance 

 the colours of thin plates, the reflected together with the trans- 

 mitted light give white. But this principle does not hold at all 

 for phenomena of diffraction, be the diffracting apertures great 

 or small ; for here the light retained by the opake parts is white, 

 like the incident. Hence, as has been rightly observed, the dif- 

 fracted would, collected, also give white. To this end it would 

 be necessary that for an aperture in the form of a slit the integral 



'sin irb\~ l sin ^\r\ 2 , 



which for each colour represents the total intensity of the dif- 

 fracted light, should be independent of \, which is clearly not 

 the case. Hence the above objection against the phenomena of 

 diffraction cannot be sustained. 



The facts enumerated in the preceding paragraphs, and conceived 

 as phenomena of diffraction, have led Babinet to propound as a 

 postulate the principle that rays of short are more readily destroyed 

 than those of longer wave-length by obstacles which are not of a 

 specific nature — that is, by fine corpuscles, no matter of what they 

 consist.- This principle is adduced in Billet, Traite d } Optique 

 Physique, vol. i. p. 168, as " Babinet' s principle." The present 

 paper, as well as that previously cited, may be regarded as 

 attempts to connect this principle of Babinet, which had not 

 hitherto been demonstrated, with the fundamental laws of the 

 undulatory theory. 



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