54 ABSORPTION AND EMISSION. 



will no longer contain the rays of different colours in the 

 same proportion as originally, and will, consequently, be co- 

 loured. 



(68) In general the colour of the emergent light is the 

 same for all thicknesses traversed, the tint only becoming 

 deeper as the thickness is augmented. But there are media 

 in which the colour itself changes with the thickness. This 

 will be seen by the foregoing formula. For if i and i 1 be the 

 original intensities of the two colours, which we shall sup- 

 pose to predominate in the transmitted light, and a and a' 

 their coefficients of transmission, we have only to suppose 

 i > i', and a < a 7 . Under such circumstances i a 9 will be at 

 first greater than i' a /e , when the thickness is small ; but it 

 will diminish more rapidly with an increase of thickness, and 

 finally become smaller. It is easily seen that the thickness 

 at which the change occurs is given by the formula 



= log- *' ~ log- *' m 

 log. a - log. a 



(69) The complete analysis of the phenomena of absorp- 

 tion is effected by interposing the absorbing medium in a 

 pencil of rays of solar light which has traversed a prism, and 

 receiving the emergent rays on a screen. For the most part 

 the spectrum of the light, which has passed through absorb- 

 ing media, thus exhibits a single maximum of absorption. 

 There are, however, media whose action upon light is more 

 complex. The blue glass which is coloured by cobalt, has 

 three such maxima, dividing the spectrum into four por- 

 tions.* The solution of chlorophyll (the colouring matter 



* This affords an easy method of obtaining a red light which is nearly homo- 

 geneous. "We have only to combine a plate of this glass with one of the red 

 glasses which transmit only the less refrangible rays of the spectrum, and we 

 shall obtain a red ray of considerable purity. 



