CHAPTER XXXIX.
CONJECTURES ON THE CAUSES OF THE CIRCULAR TRANSPARENCY TO A CERTAIN DISTANCE BELOW THE BALLOON, AND OF THE RED LIGHT FROM THE SEA AND RIVERS, WHEN SEEN ABOVE THE LEVEL OF THE SUPERIOR CLOUDS.
On the circular Transparency.
Section 222.QUERE: As Red is the heaviest and Blue the lightest Colour; and as red Rays blended at a certain Angle with blue Rays, produce Opacity: further; as red is the predominant Colour reflected from Water, while in the Form of dense Cloud, for Instance at the Rising and Setting of the Sun; and blue the Colour always reflected from the light Medium of Air or Sky; Does not this Mixture of least and most refrangible Rays, which, when aided with the intermediate primary ones, causes a Transparency near and round the Eye of a Spectator placed either on Earth or among the Clouds; produce, at a greater Distance and different Angle, such a Degree of Opacity, as actually to give the Idea of Clouds surrounding him at a Distance?
The latter Part at least is true, that Vapour and Air, which are naturally qualified to transmit red and blue, rather than any other Light, will, at a certain Angle, when blended, produce an opacity. (See the Letter sent by Newton from Cambridge to Dr. Derham, in order to be presented to the Royal Society,—in “Miscellanea Curiosa, Vol. 1, Page 109.”)
On the red Light from the Sea and Rivers.
Quere: May not the Rivers below act as a Prism; as Clouds, about Sun-set or Sun-rise, do to a Spectator on Earth, and reflect only the primary Colour red, the heaviest and least refrangible Ray?
It being also considered that Refraction cannot change the primary Colour: nor are Rays, in the Direction from below to the Zenith, refracted; tho’ seen from a rarer into a denser Medium.
Possibly, a Pencil of Rays, in coming up from the River below may be stripped or drained by the double Absorption of the Atmosphere and River, and the Colour red only, suffered to reach the Eye: “being the last to quit its Basis the Water.” (See Morgan’s Observations on the Light of Bodies, &c. &c. Phil. Trans. for the Year 1785, Part 1, Vol. 75, Chap. 91.)
