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 principle, though a simple one,
escaped him, and it was first discovered by Willebrord Snell, about
the year 1621.

Less with the view of dwelling upon the phenomenon itself than of
introducing it in a form which will render subsequently intelligible
to you the play of theoretic thought in Newton's mind, the fact of
refraction may be here demonstrated. I will not do this by drawing the
course of the beam with chalk on a black board, but by causing it to