Incidence upon it (by my Computation) would be to the Sine of its
Refraction, as 71 to 20, or 7 to 2. And therefore the Thickness of its
Particles, that they may exhibit the same Colours with those of Bubbles
of Water, ought to be less than the Thickness of the Skin of those
Bubbles in the Proportion of 2 to 7. Whence it's possible, that the
Particles of Mercury may be as little as the Particles of some
transparent and volatile Fluids, and yet reflect the white of the first
order.
Lastly, for the production of _black_, the Corpuscles must be less than
any of those which exhibit Colours. For at all greater sizes there is
too much Light reflected to constitute this Colour. But if they be
supposed a little less than is requisite to reflect the white and very
faint blue of the first order, they will, according to the 4th, 8th,
17th and 18th Observations, reflect so very little Light as to appear
intensely black, and yet may perhaps variously refract it to and fro
within themselves so long, until it happen to be stifled and lost, by
which means they will appear black in all positions of the Eye without
any transparency. And from hence may be understood why Fire, and the
more subtile dissolver Putrefaction, by dividing the Particles of
Substances, turn them to black, why small quantities of black Substances
impart their Colour very freely and intensely to other Substances to
which they are applied; the minute Particles of these, by reason of
their very great number, easily overspreading the gross Particles of
others; why Glass ground very elaborately with Sand on a Copper Plate,
'till it be well polish'd, makes the Sand, together with what is worn
off from the Glass and Copper, become very black: why black Substances
do soonest of all others become hot in the Sun's Light and burn, (which
Effect may proceed partly from the multitude of Refractions in a little
room, and partly from the easy Commotion of so very small Corpuscles;)
and why blacks are usually a little inclined to a bluish Colour. For
that they are so may be seen by illuminating white Paper by Light
reflected from black Substances. For the Paper will usually appear of a
bluish white; and the reason is, that black borders in the obscure blue
of the order described in the 18th Observation, and therefore reflects
more Rays of that Colour than of any other.
In these Descriptions I have been the more particular, because it is not
impossible but that Microscopes may at length be improved to the
discovery of the Particles of Bodies on which their Colours depend, if
147