220 FRAGMENTS OF SCIENCE. 



an interest conferred on them by previous culture. Now 

 the fact is beyond the boy's control, and so certainly is the 

 desire to know its cause. The sole question then is, 

 whether this desire is to be gratified or not. Who created 

 the fact? Who implanted the desire? Certainly not man. 

 Who then will undertake to place himself between the 

 desire and its fulfillment, 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 unprom- 

 ising 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 wisdom to rescue the boy 

 from the consequences of a wish which acts to his prej- 

 udice? Or, recognizing 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 refractive in- 

 dices. But this answer presupposes on the part of the boy 

 a knowledge of what reflection and refraction are, or re- 

 duces 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 condition the media- must possess different re- 

 fractive 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 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 tiie surprise was so great as to 

 cause a bystander to suppose that the vitreous humor had 

 been instantly dissolved. This, however, was not the case, 

 and a comparison of the refractive index of the humor 

 with that of water cleared up the whole matter. The in- 

 dices were identical, and hence the light pursued its way 

 through both as if thy f^yrccd one continuous mass. 



