ABSORPTION OF LIGHT, ETC. 41 



or absorbing medium, the less the quantity of light which 

 escapes ; and the law according to which the intensity de- 

 creases is very readily obtained by a simple ^ consideration. 

 Suppose first we had a stratum of the fluid of a certain 

 thickness, say one-tenth of an inch, and this produced a 

 certain weakening in the light, or let through a certain per- 

 centage. Now if you treated the light which came through 

 to a second stratum, also one-tenth of an inch in thickness, 

 of the same fluid, it would let through the same percentage 

 as before, and so on. When you have to deal with a mass 

 of fluid you may in imagination divide it into strata each of 

 the same thickness. Suppose here is the horizontal surface of 

 a mass of coloured fluid, which we are observing in a vertical 

 direction with white light. The effect of the fluid on light of 

 any particular kind is simply to weaken it. If we divide the 

 fluid into strata of equal thickness, in passing through the 

 first the light is weakened in a certain proportion, depending 

 on the thickness of the stratum. In passing through the 

 second stratum the same percentage of the light will be let 

 through, and so on; so that in passing from stratum to 

 stratum the intensity of the light goes on decreasing in 

 geometric proportion. That is to say, each term of the 

 series expressing the residue bears to the preceding term of 

 the series the same ratio throughout. Consequently when 

 you get far enough into the stratum the light has been so 

 much weakened that it becomes altogether invisible. Theore- 

 tically, however great the stratum, there is a quantity which 

 still gets through, but practically, after a certain time, the 

 quantity which gets through is so very small that it may be 

 regarded as nothing at all, and the light is extinguished. 

 Now the rate at which the light is so extinguished depends 

 upon the kind of light which falls upon the coloured stratum. 

 Suppose that rate to be different for different kinds of light, 

 then if there were only two kinds presented to the fluid in 

 the first instance, as it passed on and on, through this ab- 

 sorbing medium, the proportion of these two kinds of light 

 would be continually changing. For the sake of clearness I 

 will suppose there are two kinds of light to start with, blue 

 and red, and that at the beginning blue has an intensity of 

 100 and the red of 10. There is of course a great predomi- 

 nance of blue over red. Now suppose in passing through a 

 stratum of a certain thickness half the blue light is lost, and 



