Barrow, John [Editor]
Dictionarium Polygraphicum: Or, The Whole Body of Arts Regularly Digested: Illustrated with Fifty-six Copper-Plates. In Two Volumes (Band 2) — London, 1758

Page: 107
DOI Page: Citation link: 
https://digi.ub.uni-heidelberg.de/diglit/barrow1758bd2/0120
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MIR 107

and in a day's time you will have the colour dry, and as fine
as you can defire ; then put a little gum arabic into each glafs or
cup, and as much water as will moiften each of them.

Any ofthefe may be afterwards ufed with gum water ; but, if
the gum you put in at firft makes it itrong enough to glaze it,
then you need add to it only common water; and, according as
your colour is lefs or more gummed, ufe lefs or more gum wa-
ter ; for of itfelf it is a dead colour.

When you ufe this colour, touch it gently on the yellow men-
tioned, made of yellow berries, into the light fide, and, if it
wants a fhade, you may put a little vermilion upon it; but ver-
milion is too heavy to paint with, when you would illuminate
prints, becaufe it hides the {hades of the engraving; though fome-
times they had better be hidden than appear.

Some generally fhade this minium or red lead with carmine,
which gives it a fine effect, and renders it equal to the brightefl
red flower that is to be feen, leaving ftill the lights uncoloured,
only dafhing a little way into the lights with the Adinium.

When the carmine has fhaded the Minium or red lead, it may
be fhaded again with lake in the ftrongeft part, to bring it to a
deeper red.

MIRROIRS, 1 in catoptrics, is a name given to all polifhed
MIRROURS, ) bodies, which are impervious to the rays of
light, and which confequently reflect: it equally ; but, in the more
confined fenfe of the word, it is peculiarly ufed to fignify plain
or fmooth furfaces of glafs, filvered on the backfide, which ex-
hibit the images of objects oppofed to them.

The do£trine o/'Mirrours. i. Light, reflected from any Mir-
rour or looking-glafs, makes the angle of incidence equal to that
of reflection.

Hence a ray of light, falling perpendicularly on the furface of
a Mirrour or looking-glafs, will be reflected back upon itfelf, a3
is found by experience it actually does.

Therefore, from the fame point of a Mirrour, there cannot
be feveral rays reflected to the fame point, nor can the ray be
reflected into two or more points.

2. From every point of a Mirrour are reflected rays thrown on
it from every point of a radiant object. Since then rays, com-
ing from different parts of the fame object, and finking on the
fame point of the obje£t, cannot be reflected back to the fame
point; the rays which flow from different points of the fame ra-
diating object are again feparated after reflection, fo that each
point fhews whence it came.

Hence it is, that the rays reflected from Mirrours exhibit the
objects to view.

Hence alfo it appears, that rough uneven bodies muff, reflect

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