moon, divided and extinguished them. In the acta Eruditorum, printed at Leipsic[1], it will be seen that, on the 4th of March, 1728, an arc, formed by the concurrence of several clouds, was observed at Germendorf: its projection, N. E. and S. W. led to a supposition that it originated in the S. E. whence the wind then blew.
"May I be permitted to hazard another conjecture, in an age when physical novelties are rated at so high a price. I believe that the above-mentioned arc was a true iris, occasioned by the reflection of some of the stars which were setting. The demonstration is clear: it being granted that the atmosphere was charged with a multitude of atoms, and terrestrial vapours, these would, with the cold of the night, be gradually condensed, and would descend, according as their gravity should exceed that of the air in which they swam, as happens in the case of rain. As our situation was between these descending exhalations and the above-mentioned stars, the former of which were in front of us, and the latter at our side, it follows that the centre of the one, or of the other, would, according as we varied our position, coincide with our optical axes. This hypothesis being admitted, all the luminous rays with which the constellation invested the vapours, would fall on the eye of the spectator at the same angle, equal to that of its incidence. The objects which are seen at an identical angle, appear to be at an equal distance; but this equal distance could not have been verified, unless the atoms which divided the valley had represented an arc, according to the principles of optics and geo-
- ↑ December, 1730.
