500 BARON VON VVREDE ON THE ABSORPTION OF LIGHT 



I have already made the preparations necessary in order to prove by 

 experiment the identity of the phaenomena of absorption and those 

 which must result from the hypotheses assumed by me for their expla- 

 nation. The formulae which are required for such an experiment T will 

 now analyse. I have already proved that when light of all wave- 

 lengths traverses a medium which causes a retardation c, all species of 

 light whose half wave-length is 



C C C C C ^ _L 



^' P "5' T' F' ■ ' * ' 2m- 1' 2m + 1 + ' * * ' 



become minima. Now in order to derive from this a formula for the 

 minima which must arise, in consequence of the retardation c, in a 

 spectrum whose external limits are a (the greatest) and )8 (the small- 

 est), I designate this number by s, and suppose that ^ a. Z. — — y 



2m — 1 



hnt^a 7 -_^, further i /3 Z S_ butlfi^ ' "^ ^ -. 



' '^2m + l ^' 2{m + s) — r ^'^^2(m + s)+l 



y 2 c 2 c c 



Hence we have 2 m — \ ^ — and 2m + \y — ox m /_ ^-i and 



a a. a. - 



-, S- 1, and consequently m = the entire number in — + 1 , 



^ a a 2 



In the same manner M-e have m ■\- s /,■ 1- \ and y-:^ — l, conse- 



quently s = entire numb, in (-75-+^)"" entire numb, in | 1 ) (1 1 ) 



If on the contrary we assume the phaenomenon of absorption as known, 

 and search for the magnitude of the retardation which causes it, we 

 must first determine in one manner or the other the wave-lengths of 

 the species of light which represent any two minima. If I call these a' 

 and /^', and the number of the intermediate absorptions 5 — 1 (i. e. s de- 

 signates the ordinal number of the minimum whose length of undulation 

 is /3', reckoned from that whose length of undulation is a'), and suppose 



£ = i a' and ^ , ^ ^ = i /3', 



2 m' -\ - 2 (m' + 5) - 1 ^ ' ' 



we have 



a' 6«' - i) = /3' (('«' + ^) - i) 



thence 



«' _ ,3' + 2' 

 and consequently 



(12) 



