48 LECTURES TO SCIENCE TE AGREES. 



Why 1 Because there is nothing behind to reflect the light. 

 Suppose I make it a little muddy by pouring into it some 

 pounded chalk, you see the blue colour immediately. Why 

 is that 1 ? You know that if powdered chalk were put into 

 water it would not colour the fluid. J>ut here each little 

 particle of uncoloured chalk reflects a small quantity of .light 

 falling upon it, so that it fulfils the same office as a mirror 

 placed behind the fluid. You may imagine that the particles 

 of chalk are so many minute mirrors capable of reflecting 

 light. If you take any one particle of chalk, say one-tenth of 

 an inch deep, in the liquid, the light from the sky falls upon 

 the fluid, it undergoes absorption in passing through that 

 first tenth of an inch, and then the portion of light which is 

 left is reflected by that little particle of chalk, and passes out. 

 again, and so, as regards that single particle, the light which 

 reaches your eye from beneath that depth has itself gone 

 through a stratum of fluid of one-fifth of an inch in thickness, 

 and accordingly you see the colours produced by selective ab- 

 sorption, that is to say, by the absorption of certain kinds of 

 light, which are more greedily devoured by the fluid than the 

 other kinds. This is what takes place in the green leaf, and 

 in the petals of flowers. Let us take the white lily. If the 

 petal of the flower had been merely a sheet of thin glass, you 

 would not have seen that white colour. There would have 

 been a little light reflected from the first surface and the back 

 surface, but the petal is really composed of a vast assemblage 

 of little cells, at each of which partial reflection takes place, 

 so that it resembles some finely-powdered glass, which would 

 form a white powder, because each little surface is capable of 

 reflecting the light, although a single sheet of glass would not 

 be white. The petal of the white lily is just in the condition 

 of the powder. It is full of little cells, full, optically speak- 

 ing, of irregularities, from each of which a portion of light 

 is reflected, so that, all kinds being reflected alike, and there 

 being nothing in the white lily to cause preferential selection 

 of "one over the other nothing to sift the light, as it were 

 you get a considerable quantity of light reflected back to 

 the eye, bat it is white. What is the difference between that 

 and the red poppy 1 The red poppy is, as it were, a white lily 

 infused with a red fluid ; there is light continually reflected 

 backwards and forwards, just as before, at the surface of the 

 cells ; but that light, in going and coming, passes through the 



