d 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 have a transparent
solid mixed with air. At every transition from solid to air, or
from air to solid, a portion of light is reflected, and this takes
place so often that the light is wholly intercepted. Thus from the
mixture of two transparent bodies we obtain an opaque one. Now the
case of the towel is precisely similar. The tissue is composed of
semi-transparent vegetable fibres, with the interstices between them
filled with air; repeated reflection takes place at the limiting
surfaces of air and fibre, and hence the towel becomes opaque like
snow or salt. But if we fill the interstices with water, we diminish
the reflection; a portion of the light is transmitted, and the
darkness of the towel is due to its increased transparency. Thus the
deportment of various minerals, such as hydrophane and tabasheer, the
transparency of tracing paper used by engineers, and many other
considerations of the highest scientific interest, are involved in the
simple enquiry of this unsuspecting little boy.

Again, take the question regarding the rising or falling of the dew--a
question long agitated, and finally set at rest by the beautiful
researches of Wells. I do not think that any boy of average
intelligence will be satisfied with the simple answer that the dew
falls. He will wish to learn how you know that it falls, and, if
acquainted with the notions of the middle ages, he may refer to the
opinion of Father Laurus, that a goose egg filled in the morning with
dew and exposed to the sun, will rise like a balloon--a swan's egg
being better for the experiment than a goose egg. It is impossible to
give the boy a clear notion of the beautiful phenomenon to which his
question refers, without first making him acquainted with the
radiation and conduction of heat. Take, for example, a blade of
grass, from which one of these orient pearls is depending.

During the day the grass, and the earth beneath it, possess a certain
amount of warmth imparted by the sun; during a serene night, heat is
radiated from the surfa