42 LECTURES TO SCIENCE TEACHERS. ' 



only half transmitted, and that ninety per cent, of the red 

 light is transmitted. Then after passing through the first 

 stratum the intensities will be respectively 50 and 9 ; after 

 passing the second stratum of the same thickness the intensities 

 will be 25 and 8'1 ; after the third 12'5 and 7'3 ; after the 

 fourth 6 - 2 and 6 '6, or about equal; but afcer passing through 

 the next stratum they will be 3'1 and 5'8 ; so that 

 although the quantity of red light was so much smaller lo 

 begin with, the red is more lasting, and in light which has 

 passed through five of these strata the red now predominates 

 over the blue. Passing through another stratum, the in- 

 tensity of the blue is reduced to 1*5, whilst the red is 5*2, 

 and so on ; so that you see both kinds of light are weakened 

 but the proportion to one another is continually changing. 

 That is a general explanation of what takes place in a fluid 

 such as this [exhibiting an alkaline solution of archil]. I 

 may mention that in almost all coloured fluids there is a con- 

 tinual change in the colour according to the thickness of 

 the stratum of liquid, or, which will come to the same thing, 

 according to the strength of the solution. For the sake of 

 simplicity of explanation, I supposed there were only two 

 kinds of light to deal with, which I called red and blue, but 

 in point of fact when the fluid has white light thrown upon 

 it we have an infinite number of kinds of light, and all shades 

 of refrangibility, and each shade of refrangibility must be 

 considered by itself. If we take a certain stratum of a 

 coloured liquid or glass, or whatever it is, then after passing 

 through that stratum the light will be weakened in a pro- 

 portion which changes continuously in passing from one end 

 of the spectrum to the other. The mode in which the total 

 light passing through the stratum is made up may very con- 

 veniently be represented to the eye by a construction given 

 by Sir John Herschel in his treatise on light. Suppose you 

 take a horizontal line and lay distances along that line, or 

 abscissae, representing the places of the kinds of light which 

 you have under consideration in some standard spectrum, and 

 let lines drawn vertically, or ordinates, represent the intensity 

 of the particular kind of light. If you care merely to know 

 how the quantities go on changing as the light passes deeper 

 and deeper into the absorbing fluid or glass, it will be 

 simplest, to take the original intensity as unity throughout, 

 although we know very well that the different parts of the 



