Experience shows us that the air must have darkness beyond it and
yet it appears blue. If you produce a small quantity of smoke from
dry wood and the rays of the sun fall on this smoke, and if you then
place behind the smoke a piece of black velvet on which the sun does
not shine, you will see that all the smoke which is between the eye
and the black stuff will appear of a beautiful blue colour. And if
instead of the velvet you place a white cloth smoke, that is too
thick smoke, hinders, and too thin smoke does not produce, the
perfection of this blue colour. Hence a moderate amount of smoke
produces the finest blue. Water violently ejected in a fine spray
and in a dark chamber where the sun beams are admitted produces
these blue rays and the more vividly if it is distilled water, and
thin smoke looks blue. This I mention in order to show that the
blueness of the atmosphere is caused by the darkness beyond it, and
these instances are given for those who cannot confirm my experience
on Monboso.