I say that the blueness we see in the atmosphere is not intrinsic
colour, but is caused by warm vapour evaporated in minute and
insensible atoms on which the solar rays fall, rendering them
luminous against the infinite darkness of the fiery sphere which
lies beyond and includes it. And this may be seen, as I saw it by
any one going up [Footnote 5: With regard to the place spoken of as
_M'oboso_ (compare No. 301 line 20) its identity will be discussed
under Leonardo's Topographical notes in Vol. II.] Monboso, a peak of
the Alps which divide France from Italy. The base of this mountain
gives birth to the four rivers which flow in four different
directions through the whole of Europe. And no mountain has its base
at so great a height as this, which lifts itself almost above the
clouds; and snow seldom falls there, but only hail in the summer,
when the clouds are highest. And this hail lies [unmelted] there, so
that if it were not for the absorption of the rising and falling
clouds, which does not happen twice in an age, an enormous mass of
ice would be piled up there by the hail, and in the middle of July I
found it very considerable. There I saw above me the dark sky, and
the sun as it fell on the mountain was far brighter here than in the
plains below, because a smaller extent of atmosphere lay between the
summit of the mountain and the sun. Again as an illustration of the
colour of the atmosphere I will mention the smoke of old and dry
wood, which, as it comes out of a chimney, appears to turn very
blue, when seen between the eye and the dark distance. But as it
rises, and comes between the eye and the bright atmosphere, it at
once shows of an ashy grey colour; and this happens because it no
longer has darkness beyond it, but this bright and luminous space.
If the smoke is from young, green wood, it will not appear blue,
because, not being transparent and being full of superabundant
moisture, it has the effect of condensed clouds which take distinct
lights and shadows like a solid body. The same occurs with the
atmosphere, which, when overcharged with moisture appears white, and
the small amount of heated moisture makes it dark, of a dark blue
colour; and this will suffice us so far as concerns the colour of
the atmosphere; though it might be added that, if this transparent
blue were the natural colour of the atmosphere, it would follow that
wherever a larger mass air intervened between the eye and the
element of fire, the azure colour would be more intense; as we see
in blue glass and in sapphires, which are darker in proportion as
they are larger. But the atmosphere in such circumstances behaves in
an opposite manner, inasmuch as where a greater quantity of it lies
between the eye and the sphere of fire, it is seen much whiter. This
occurs towards the horizon. And the less the extent of atmosphere
between the eye and the sphere of fire, the deeper is the blue
colour, as may be seen even on low plains. Hence it follows, as I
say, that the atmosphere assumes this azure hue by reason of the
particles of moisture which catch the rays of the sun. Again, we may
note the difference in particles of dust, or particles of smoke, in
the sun beams admitted through holes into a dark chamber, when the
former will look ash grey and the thin smoke will appear of a most
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_.]
301.
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.
302.
When the smoke from dry wood is seen between the eye of the
spectator and some dark space [or object], it will look blue. Thus
the sky looks blue by reason of the darkness beyond it. And if you
look towards the horizon of the sky, you will see the atmosphere is
not blue, and this is caused by its density. And thus at each
degree, as you raise your eyes above the horizon up to the sky over
your head, you will see the atmosphere look darker [blue] and this
is because a smaller density of air lies between your eye and the
[outer] darkness. And if you go to the top of a high mountain the
sky will look proportionately darker above you as the atmosphere
becomes rarer between you and the [outer] darkness; and this will be
more visible at each degree of increasing height till at last we
should find darkness.
That smoke will look bluest which rises from the driest wood and
which is nearest to the fire and is seen against the darkest
background, and with the sunlight upon it.
303.
A dark object will appear bluest in proportion as it has a greater
mass of luminous atmosphere between it and the eye. As may be seen
in the colour of the sky.
304.
The atmosphere is blue by reason of the darkness above it because
black and white make blue.
305.
In the morning the mist is denser above than below, because the sun
draws it upwards; hence tall buildings, even if the summit is at the
same distance as the base have the summit invisible. Therefore,
also, the sky looks darkest [in colour] overhead, and towards the
horizon it is not blue but rather between smoke and dust colour.
The atmosphere, when full of mist, is quite devoid of blueness, and
only appears of the colour of clouds, which shine white when the
weather is fine. And the more you turn to the west the darker it
will be, and the brighter as you look to the east. And the verdure
of the fields is bluish in a thin mist, but grows grey in a dense
one.
The buildings in the west will only show their illuminated side,
where the sun shines, and the mist hides the rest. When the sun
rises and chases away the haze, the hills on the side where it lifts
begin to grow clearer, and look blue, and seem to smoke with the
vanishing mists; and the buildings reveal their lights and shadows;
through the thinner vapour they show only their lights and through
the thicker air nothing at all. This is when the movement of the
mist makes it part horizontally, and then the edges of the mist will
be indistinct against the blue of the sky, and towards the earth it
will look almost like dust blown up. In proportion as the atmosphere
is dense the buildings of a city and the trees in a landscape will
look fewer, because only the tallest and largest will be seen.
Darkness affects every thing with its hue, and the more an object
differs from darkness, the more we see its real and natural colour.
The mountains will look few, because only those will be seen which
are farthest apart; since, at such a distance, the density increases
to such a degree that it causes a brightness by which the darkness
of the hills becomes divided and vanishes indeed towards the top.
There is less [mist] between lower and nearer hills and yet little
is to be distinguished, and least towards the bottom.
306.
The surface of an object partakes of the colour of the light which
illuminates it; and of the colour of the atmosphere which lies
between the eye and that object, that is of the colour of the
transparent medium lying between the object and the eye; and among
colours of a similar character the second will be of the same tone
as the first, and this is caused by the increased thickness of the
colour of the medium lying between the object and the eye.