54 SCIENCE FROM AN EASY CHAIR 



reflected from it to your eye. It is now opaque white 

 or colourless, with, perhaps, a faint tinge of blue. This 

 change is due as was shown by the experiments of the 

 late Professor Tyndall upon a variety of clouds and 

 vapours to the cooling of the smoke and the increased 

 size of the floating particles which coalesce as the tem- 

 perature falls. The larger particles reflect white light, 

 and the cloud is no longer blue. A cloud formed by the 

 finest particles gives the strongest blue to the light 

 reflected from it, and it is to this property of the finest 

 particles of water-cloud floating in our atmosphere that 

 the blue colour of the sky is due. 



No doubt the question arises, " Why do clouds of the 

 finest particles reflect a predominant amount of blue light 

 rather than yellow or green or red ? " That question is 

 answered by mathematicians in accordance with what is 

 ascertained as to the nature and properties of light, but it 

 would require a long treatise to put those matters even in 

 outline before the reader. We may in the meanwhile 

 accept the conclusions of the physicists, and interest our- 

 selves in seeing how they apply to some of the concrete 

 facts about colour in Nature. 



There are other instances of " blueness " due to the 

 reflection of light from a cloud of excessively minute 

 particles besides that of the azure sky and the blue, 

 curling smoke of a wood fire. A familiar instance is the 

 blueness of translucent bodies, such as the " white " of a 

 boiled plover's egg, especially when a bit of it is placed on 

 a dead-black ground. The bluish appearance of watered 

 London milk is another instance. These bodies look blue 

 owing to the fine, colourless particles suspended in them, 

 which act on light in the same way as do the fine particles 

 of newly-produced smoke. Another very interesting case 

 is the blue colour of the iris of the eye of man and other 

 animals. It is not due to any blue pigment, but to a 



