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beautiful blue; and it may be seen again in in the dark shadows of
distant mountains when the air between the eye and those shadows
will look very blue, though the brightest parts of those mountains
will not differ much from their true colour. But if any one wishes
for a final proof let him paint a board with various colours, among
them an intense black; and over all let him lay a very thin and
transparent [coating of] white. He will then see that this
transparent white will nowhere show a more beautiful blue than over
the black--but it must be very thin and finely ground.
[Footnote 7: reta here has the sense of malanno.]
3
01.
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
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