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.

[Sidenote: 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.”)

[Sidenote: 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.)