British Association, 115 



a portion of each wave encounters some of the particles, and is re- 

 flected backwards, then forwards again, and emerges along with the 

 direct ray ; and from the retardation it has undergone it may interfere 

 80 as to produce darkness, if the retardation amount to an odd mul- 

 tiple of the half-wave length (A), but brightness if an even multiple 

 of that quantity. These effects may be confounded together in white 

 light, and by prismatic analysis they will be seen as dark bands. 



The author investigated a formula for the intensity of the light so 

 resulting. It was deduced from the ordinary view of undulations, and 

 brought into a form including certain constant terms, together with 



2 6 

 the factor cos 2 it — , which is so involved, that the intensity is a 



A 



maximum when the cosine is = + 1 or when 2 tt — is an even mul- 



A 



tiple of a semicircumference j and a minimum when the cosine == — 1 , 

 or when the arc is an odd multiple of a semicircumference. Hence, 



if the medium be such that 2 6 = - for any ray, that ray will be de- 



ficient. If 2 5 be less than the value of - for the violet ray, which is 



its least value, there will be no absorption : if greater, some ray will 

 be at a minimum. Let us suppose 2 6 = w A, then passing from one 

 end of the spectrum to the other, there will be changes of intensity 

 dependent on the changes of the cosine between the limits cos litn 



and cos 2*71 — ; maxima when cos= + 1, and minima when — I : 



the number depending on (n,) that is, on (6), which may be supposed 

 as large as we please. 



This investigation applying to a simple medium, the author showed 

 that the expression for a compound of several media, with different 

 values of (6), will still preserve the condition of depending on the 

 changes of the cosine, and each medium will retain its own set of 

 maxima and minima, which will be superposed in the spectrum. 



Thus far the successive reflections had been considered as taking 

 place only between two sets of particles or reflecting surfaces : the 

 case was then investigated where several such were taken into ac- 

 count, and a formula resulted which was more complex, but whose 

 maxima and minima depended on exactly the same conditions. 



The author showed that the more regular phaenomena of absorption 

 are completely explained by this hypothesis, and, in one instance, 

 even proceeded with success to a numerical comparison. He pointed 

 out also an experimental imitation of the supposed process, which was 

 perfectly successful.* 



Sir David Brewster conceived that the theory of Von Wrede was 

 entirely inadmissible. He stated many cases of absorption where 

 there was not an appearance of reflection, as in the case of nitrous gas, 

 which by mere changes of temperature became as black and as im- 



* A translation of the original memoir on this subject by M. Von Wrede 

 will be found in the Scientific Memoirs, vol. i. p. 477. 



Q2 



