nd to the other, which
scattered some of the Sun's Light irregularly, but had no sensible
Effect in increasing the Length of the coloured Spectrum. For I tried
the same Experiment with other Prisms with the same Success. And
particularly with a Prism which seemed free from such Veins, and whose
refracting Angle was 62-1/2 Degrees, I found the Length of the Image
9-3/4 or 10 Inches at the distance of 18-1/2 Feet from the Prism, the
Breadth of the Hole in the Window-shut being 1/4 of an Inch, as before.
And because it is easy to commit a Mistake in placing the Prism in its
due Posture, I repeated the Experiment four or five Times, and always
found the Length of the Image that which is set down above. With another
Prism of clearer Glass and better Polish, which seemed free from Veins,
and whose refracting Angle was 63-1/2 Degrees, the Length of this Image
at the same distance of 18-1/2 Feet was also about 10 Inches, or 10-1/8.
Beyond these Measures for about a 1/4 or 1/3 of an Inch at either end of
the Spectrum the Light of the Clouds seemed to be a little tinged with
red and violet, but so very faintly, that I suspected that Tincture
might either wholly, or in great Measure arise from some Rays of the
Spectrum scattered irregularly by some Inequalities in the Substance and
Polish of the Glass, and therefore I did not include it in these
Measures. Now the different Magnitude of the hole in the Window-shut,
and different thickness of the Prism where the Rays passed through it,
and different inclinations of the Prism to the Horizon, made no sensible
changes in the length of the Image. Neither did the different matter of
the Prisms make any: for in a Vessel made of polished Plates of Glass
cemented together in the shape of a Prism and filled with Water, there
is the like Success of the Experiment according to the quantity of the
Refraction. It is farther to be observed, that the Rays went on in right
Lines from the Prism to the Image, and therefore at their very going out
of the Prism had all that Inclination to one another from which the
length of the Image proceeded, that is, the Inclination of more than two
degrees and an half. And yet according to the Laws of Opticks vulgarly
received, they could not possibly be so much inclined to one another.[D]
For let EG [_Fig._ 13. (p. 27)] represent the Window-shut, F the hole
made therein through which a beam of the Sun's Light was transmitted
into the darkened Chamber, and ABC a Tri