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
mark its own white track before you. A shallow circular vessel (RIG,
fig. 4), half filled with water, rendered slightly turbid by the
admixture of a little milk, or the precipitation of a little mastic,
is placed with its glass front vertical. By means of a small plane
reflector (M), and through a slit (I) in the hoop surrounding the
vessel, a beam of light is admitted in any required direction. It
impinges upon the water (at O), enters it, and tracks itself through
the liquid in a sharp bright band (O G).
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