288 FRAGMENTS OF SCIENCE. 



Or, recognising the propriety of the question, how 

 shall we answer it? It is impossible to answer it with- 

 out reference to the laws of optics without making 

 the boy to some extent a natural philosopher. You 

 may say that the effect is due to the reflection of light 

 at the common surface of two media of different re- 

 fractive indices. But this answer presupposes on the 

 part of the boy a knowledge of what reflection and 

 refraction are, or reduces you to the necessity of ex- 

 plaining them. 



On looking more closely into the matter, we find 

 that our wet towel belongs to a class of phenomena 

 which have long excited the interest of philosophers. 

 The towel is white for the same reason that snow is 

 white, that foam is white, that pounded granite or 

 glass is white, and that the salt we use at table .is 

 white. On quitting one medium and entering another, 

 a portion of light is always reflected, but on this condi- 

 tion the media must possess different refractive in- 

 dices. Thus, when we immerse a bit of glass in water, 

 light is reflected from the common surface of both, and 

 it is this light which enables us to see the glass. But 

 when a transparent solid is immersed in a liquid of the 

 same refractive index as itself, it immediately disap- 

 pears. I remember once dropping the eyeball of an 

 ox into water; it vanished as if by magic, with the 

 exception of the crystalline lens, and the surprise was 

 so great as to cause a bystander to suppose that the 

 vitreous humour had been instantly dissolved. This, 

 however, was not the case, and a comparison of the 

 refractive index of the humour with that of water 

 cleared up the whole matter. The indices were identi- 

 cal, and hence the light pursued its way through both 

 as if they formed one continuous mass. 



In the case of snow, powdered quartz, or salt, we 



