288 FBAGMENTS OF SCIENCE. 



recognising the propriety of the question, how shall we 

 answer it ? It is impossible to answer it without 

 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 

 refractive 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 

 explaining 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 indices. 

 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 disappears. 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 identical, 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 



