304 FRAGMENTS OF SCIENCE 



then is, whether this desire is to be gratified or not. 

 Who created the fact? Who implanted the desire? Cer- 

 tainly not man. Who then will undertake to place him- 

 self between the desire and its fulfilment, and proclaim a 

 divorce between them? Take, for example, the case of 

 the wetted towel, which at first sight appears to be one 

 of the most unpromising questions in the list. Shall we 

 tell the proposer to repress his curiosity, as the subject 

 is improper for him to know, and thus interpose our wis- 

 dom to rescue the boy from the consequences of a wish 

 which acts to his prejudice? Or, recognizing the propri- 

 ety of the question, how shall we answer it? It is im- 

 possible 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 me- 

 dium and entering another, a portion of light is always 

 reflected, but on this condition 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 



