Proportion.
Now, considering that these grey and dun Colours may be also produced by
mixing Whites and Blacks, and by consequence differ from perfect Whites,
not in Species of Colours, but only in degree of Luminousness, it is
manifest that there is nothing more requisite to make them perfectly
white than to increase their Light sufficiently; and, on the contrary,
if by increasing their Light they can be brought to perfect Whiteness,
it will thence also follow, that they are of the same Species of Colour
with the best Whites, and differ from them only in the Quantity of
Light. And this I tried as follows. I took the third of the
above-mention'd grey Mixtures, (that which was compounded of Orpiment,
Purple, Bise, and _Viride ris_) and rubbed it thickly upon the Floor of
my Chamber, where the Sun shone upon it through the opened Casement; and
by it, in the shadow, I laid a Piece of white Paper of the same Bigness.
Then going from them to the distance of 12 or 18 Feet, so that I could
not discern the Unevenness of the Surface of the Powder, nor the little
Shadows let fall from the gritty Particles thereof; the Powder appeared
intensely white, so as to transcend even the Paper it self in Whiteness,
especially if the Paper were a little shaded from the Light of the
Clouds, and then the Paper compared with the Powder appeared of such a
grey Colour as the Powder had done before. But by laying the Paper where
the Sun shines through the Glass of the Window, or by shutting the
Window that the Sun might shine through the Glass upon the Powder, and
by such other fit Means of increasing or decreasing the Lights wherewith
the Powder and Paper were illuminated, the Light wherewith the Powder is
illuminated may be made stronger in such a due Proportion than the Light
wherewith the Paper is illuminated, that they shall both appear exactly
alike in Whiteness. For when I was trying this, a Friend coming to visit
me, I stopp'd him at the Door, and before I told him what the Colours
were, or what I was doing; I asked him, Which of the two Whites were the
best, and wherein they differed? And after he had at that distance
viewed them well, he answer'd, that they were both good Whites, and that
he could not say which was best, nor wherein their Colours differed.
Now, if you consider, that this White of the Powder in the Sun-shine was
compounded of the Colours which the component Powders (Orpiment, Purple,
Bise, and _Viride ris_) have in the same Sun-shine, you must
acknowledge by this Experiment, as well as by the former, that perfect
Whiteness may be compounded of Colours.
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