Iris was broader than the interior, but so faint, especially on the blue
side, that I could not measure its breadth distinctly. At another time
when both Bows appeared more distinct, I measured the breadth of the
interior Iris 2 Gr. 10 , and the breadth of the red, yellow and green in
the exterior Iris, was to the breadth of the same Colours in the
interior as 3 to 2.
This Explication of the Rain-bow is yet farther confirmed by the known
Experiment (made by _Antonius de Dominis_ and _Des-Cartes_) of hanging
up any where in the Sun-shine a Glass Globe filled with Water, and
viewing it in such a posture, that the Rays which come from the Globe to
the Eye may contain with the Sun's Rays an Angle of either 42 or 50
Degrees. For if the Angle be about 42 or 43 Degrees, the Spectator
(suppose at O) shall see a full red Colour in that side of the Globe
opposed to the Sun as 'tis represented at F, and if that Angle become
less (suppose by depressing the Globe to E) there will appear other
Colours, yellow, green and blue successive in the same side of the
Globe. But if the Angle be made about 50 Degrees (suppose by lifting up
the Globe to G) there will appear a red Colour in that side of the Globe
towards the Sun, and if the Angle be made greater (suppose by lifting
up the Globe to H) the red will turn successively to the other Colours,
yellow, green and blue. The same thing I have tried, by letting a Globe
rest, and raising or depressing the Eye, or otherwise moving it to make
the Angle of a just magnitude.
I have heard it represented, that if the Light of a Candle be refracted
by a Prism to the Eye; when the blue Colour falls upon the Eye, the
Spectator shall see red in the Prism, and when the red falls upon the
Eye he shall see blue; and if this were certain, the Colours of the
Globe and Rain-bow ought to appear in a contrary order to what we find.
But the Colours of the Candle being very faint, the mistake seems to
arise from the difficulty of discerning what Colours fall on the Eye.
For, on the contrary, I have sometimes had occasion to observe in the
Sun's Light refracted by a Prism, that the Spectator always sees that
Colour in the Prism which falls upon his Eye. And the same I have found
true also in Candle-light. For when the Prism is moved slowly from the
Line which is drawn directly from the Candle to the Eye, the red appears
first in the Prism and then the blue, and therefore each of them is seen
when it falls upon the Eye. For the red passes over the Eye first, and
then the blue.
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