Essay Newton

Introduction

While reading Newton's Philosophy of Nature, I was particularly struck by the essay "The New Theory About Light and Colors" on p. 68.

I procured me a triangular glass prism, to try therewith the celebrated phenomena of colors.
- p. 68

A brief summary of the essay:

Initial experiments with the prism:

Newton procured himself a prism
He procured a dark room, made a small hole for light to come through, placed prism in front
Observed that the projection on the opposite wall was oblong, not circular; L = 5 W
Tested effect of variables: thickness of prism, brightness of light, prism inside versus outside the window
Observed: these had no effect on the shape of the projection - still oblong, still L = 5 W
Tested whether it was defects in the prism by putting 2 prisms next to each other (second one "reverses" refraction of first)
Observed that 1st prism turned a circular hole into an oblong shape; 2nd prism turned oblong shape into orbicircular with same regularity as if there were no prisms
Deduced there were no imperfections in the prism glass/surface causing the oblong shape

Quantitative approach:

Began to measure various aspects of configuration
Distance from hole to prism, utmost length, breadth, diameter of hole, angle of rays, vertical angle of prism, angle of refraction on both sides
Quantified difference between L and W (L = 5 W)
Computed refractive power of glass from Law of Sines
For 2 rays coming from opposite sides of sun's disc, determined angle they would form, verified measurements with calculation
Still no explanation for L = 5 W
Playing with prism angle, rotated it on its axis (obliqueness to rays of sun)
Observed no effect
The difference of incidence angles of rays coming from opposite sides of sun's disc, both show that W checks out - but L = 5 W does not
Hypothesis: rays, after passing through prism, move in curved trajectories
Analogy: tennis ball, when struck and rotating, has curvature induced by spin, higher drag on one side of the ball causing arc, if light similarly had drag it would also curve
Observed no curvature of light, just that difference between L and the diameter of the hole was proportional to distance between them (projected shape gets larger as the hole gets further away)

Experimentum Crucis:

2 boards, 2 prisms:
Window slit --> prism --> board with hole --> board with hole --> prism
Rotating prism 1 about its axis and casting different parts of the projection onto the first board, those rays then passed through the second board and through the second prism and were reflected on the wall behind.
Experiments focused on where on the wall the rays passing through prism 1 and prism 2 were projected