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l 4 8 FRAGMEN TS OF SCIENCE.

with great distinctness which vessel contains the largest
particles. The retina is very sensitive to differences of
light, when, as here, the eye is in comparative darkness,
and when the quantities of wave-motion thrown against
the retina are small. The larger particles declare them-
selves by the greater whiteness of their scattered light.
Call now to mind the observation, or effort at observation,
made by our President, when he failed to distinguish the
particles of, mastic in Briicke’s medium, and when you have
done so follow me. I permitted a beam of light to act
upon a certain vapor. In two minutes the azure appeared,
but at the end of fifteen minutes it had not ceased to be
azure. After fifteen minutes, for example, its color, and
some other phenomena, pronounced it to be a blue of dis-
tinctly smaller particles than those sought for in vain by
Mr. Huxley. These particles, as already stated, must have
been less than Twin—6th of an inch in diameter. And now
I want you to submit to your imagination the following
question: Here are particles which have been growing
continually for fifteen minutes, and at the end of that time
are demonstrably smaller than those which defied the mi-
croscope of Mr. Huxley: what must have been the size of
these particles at the beginning of their growth? What
notion can you form of the magnitude of such particles ?
The distances of stellar space give us simply a bewildering
sense of vastness without leaving any distinct impression
on the mind, and the magnitudes with which we have here
to do bewilder us equally in the opposite direction. We.
are dealing with infinitesimals compared with which the
test objects of the microscope are literally immense.

From their perviousness to stellar light and other con-
siderations, Sir John Herschel drew some startling conclu-
sions regarding the density and weight of comets. You
know that these extraordinary and mysterious bodies some.
times throw out tails 100,000,000 of miles in length, and

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