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 the 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 they formed one continuous rna,ss, 
