nstrated. Helmholtz long ago employed
this method of rendering evident to his students the classical
experiments of Du Bois Raymond on animal electricity; while in Sir
William Thomson's reflecting galvanometer the principle receives one
of its latest and most important applications.

Sec. 4. _The Refraction of Light. Total Reflection._

For more than a thousand years no step was taken in optics beyond this
law of reflection. The men of the Middle Ages, in fact, endeavoured,
on the one hand, to develop the laws of the universe _a priori_ out of
their own consciousness, while many of them were so occupied with the
concerns of a future world that they looked with a lofty scorn on all
things pertaining to this one. Speaking of the natural philosophers of
his time, Eusebius says, 'It is not through ignorance of the things
admired by them, but through contempt of their useless labour, that we
think little of these matters, turning our souls to the exercise of
better things.' So also Lactantius--'To search for the causes of
things; to inquire whether the sun be as large as he seems; whether
the moon is convex or concave; whether the stars are fixed in the sky,
or float freely in the air; of what size and of what material are the
heavens; whether they be at rest or in motion; what is the magnitude
of the earth; on what foundations is it suspended or balanced;--to
dispute and conjecture upon such matters is just as if we chose to
discuss what we think of a city in a remote country, of which we never
heard but the name.'

As regards the refraction of light, the course of real inquiry was
resumed in 1100 by an Arabian philosopher named Alhazen. Then it was
taken up in succession by Roger Bacon, Vitellio, and Kepler. One of
the most important occupations of science is the determination, by
precise measurements, of the quantitative relations of phenomena; the
value of such measurements depending greatly upon the skill and
conscientiousness of the man who makes them. Vitellio appears to have
been both skilful and conscientious, while Kepler's habit was to
rummage through the observations of his predecessors, to look at them
in all lights, and thus distil from them the principles which united
them. He had done this with the astronomical measurements of Tycho
Brahe, and had extracted from them the celebrated 'laws of Kepler.' He
did it also with Vitellio's measurements of refraction. But in this
case he was not successful. The pri