only by the white or colourless particles. They con- trine that the rays of light are reflected, refracted, and
sist of pellucid media, throughont which white or co- inflected, by one and the same principle acting vario
lourless opaque particles are dispersed. The latter are ously in various circumstances."
disposed at such distances from each other, that some The most remarkable part of Mr Delaval's doctrine
of the incident rays of light are capable of passing is that concerning the metals ; for the better under.
through the intervals which intercede them, and thus standing of which we sliall premise a short abstract of
are transmitted through the semipellucid mass. Some his general doctrine concerning white bodies, and the
sorts of rays penetrate through such masses, while manner in which light is reflected by them. “ All the or the
others, which differ from them in their refrangibility, earths, (he observes), which in their natural state are manner in
are reflected by the light or colourless particles; and of a pure white, constitute transparent colourless media which lig

!
from thence are transmitted through the pellucid part when vitrified with proper fluxes, or when dissolved in is reflecte
of the medium which intervenes between the reflecting colourless menstrua; and the saline masses obtainable bodies.
particles and the anterior surface of the mass. On the from their solutions are transparent and colourless,
same principle our author explains the blue colour of while they retain the water which is essential to their

the sky, the green colour of the sea, and other natural crystallization, and are not flawed or reduced to powa
43 phenomena : and from his numerous experiments on der ; bat after their pores and interstices are opened
How co- this subject at last conclades, " that the power by which in such a manner as to admit the air, they become
lours are
shown by the several rays of light are transmitted through differ, then white and opaque by the entrance of that rare
transmitted ent media is inherent in the particles themselves, and medium. The earthy particles which form the solid
light. therefore is not confined to the surfaces of such media. parts of bodies generally exceed the other in density;

For if the transmissive force was exerted at the surface consequently these particles, when contiguous to the
only, the thinner plates of coloured substances would rare media already mentioned, must reflect the rays of
act upon the rays as powerfully as thicker masses. But light with a force proportionate to their density. The
it appears from experiment, that in proportion as the reflective power of bodies does not depend merely upon
rays pass through different thicknesses of coloured me.

their excess of density, but upon their difference of
dia, they exhibit colours differing not only in degree, - density with respect to the surrounding media. Trans-
but freqnently in species also.

parent colourless particles, whose density is greatly in-
“ The sun's light, by which bodies are illuminated, ferior to that of the media they come between, also
consists of all the rays of which a white light is com- powerfully reflect all sorts of rays, and thereby become
pounded. These rays, in their entire and undivided white. Of this kind are the air or other rare Auids
state, are incident upon the opaque particles of semi- which occupy the interstices of liquors; and in gene.
pellucid substances, and upon the colouring particles ral of all denser media in whose interstices such rare
of transparent coloured substances, whenever these particles are admitted.
media are exposed to the light. When the rays accede Hence we may conclude, that white opaque bodies
to the opaque particles of semipellucid substances, are constituted by the union or contiguity of two or
some sorts of them are reflected back from the ante- more transparent colourless media differing consider-
rior surface of these particles : the other sorts of rays, ably from each other in their reflective powers. Of
which are not reflected back, are diverted from the these substances we have examples in froth, emulsions,
direction which is opposite to the anterior surface or other imperfect combinations of pellucid liquors,
of the opaque particles, and passing through the inter- milk, snow, calcined or pulverized salts, glass of cry-
vals between the particles, are transmitted through stal reduced to powder, white earths, paper, linen, and
the mass.

even those metals which are called white by mineralo-
“ When the rays are incident upon the particles of gists and chemists : for the metals just mentioned do
transparent coloured bodies, none of them are reflected not appear white unless their surfaces be rough; as in
back; because the colouring particles are not endowed that case only there are interstices on their surfaces suf-
with
any
reflective

power ;

but some of the rays are ficient to admit the air, and thus make a reflection of a
either stopped at the interior surface of the particles, white and vivid light.
or are diverted into such directions as render them in- “ But the polished surfaces of metallic mirrors re-
capable of passing towards the further side of the mass; flect the incident rays equably and regularly, accord-
and consequently such rays cannot be transmitted. Thé ing to their several angles of incidence; so that the
rays which are thus intercepted or dispersed, are reflected rays do not interfere with each other, but
transmitted in the same manner as those which pass remain separate and unmixed, and therefore distinctly
through semipellucid media. Thus it is evident, that exhibit their several colours. Hence it is evident,
the coloured rays which are transmitted through semi- that white surfaces cannot act upon the light ag mir-
pellucid substances, are inflected by the opaque particles; rors; because all the rays which are reflected from
and those which are transmitted through transparent- them are blended in a promiscuous and disorderly
coloured substances are inflected by the colouring par-

45
ticles. From the preceding observations likewise it ap- “ The above mentioned phenomena give much in- of the
pears, that the particles of coloured media inflect the sight into the nature and cause of opacity: as they cause of
several sorts of rays according to the several sizes and clearly show, that even the rarest transparent colour- opacity.
densities of the particles ; also in proportion to the in- less substances, when their surfaces are adjacent to me-
flammability of the media which owe their colour to dia differing greatly from them in refractive power,
them; and it is manifest that the transmission of co- may thereby acquire a perfect opacity, and may assume
loured rays depends opon their inflection. All these ob- a resplendency and hue so similar to that of white me-
servations are conformable to Sir Isaac Newton's doc- tals, that the rarer pellucid substances cannot by the
2

sight

manner.

a

Plate CXLV.

Fig. 1.

aurora, &c.

a

eolour of those three. For the yellow and blue, if placed, a bright violet will be produced: and by the
they are equal in quantity, will draw the intermediate red and yellow, the several shades of orange.
green equally toward them, and keep it, as it were, in If, instead of placing these glasses in a similar posi. Fig. 3.
equilibrio, that it verge not more to the one than to tion, you place the side AB of the yellow glass against
the other. To this compound green there may be the side BD of the blue, you will see all the various
added some red and violet ; and yet the green will not greens that are produced by nature (E); if the blue
immediately cease, but grow less vivid ; till by adding and red glasses be placed in that manner, you will have
mere red and violet it will become more diluted ; and all the possible varieties of purples, violets, &c.; and,
at last, by the prevalence of the added colours, it will lastly, if the red and orange glasses be so placed, there
be overcome, and turned into some anomalous colour. will be all the intermediate colours, as the marygold,
If the sun's white, composed of all kinds of rays,

be
added to any homogeneous colour, that colour will not
vanish, nor change its species, but be diluted; and by IV. By means of the three primary colours, red, yellow,
adding more white, it will become continually more

and blue, together with light and shade, to produce diluted. Lastly, if red and violet be mixed, there will

all the gradations of the prismatic colours. be generated, according to their various proportions, various purples, such as are not like, in appearance, On seven square panes of glass, paste papers that to the colour of any homogeneous light; and of these are painted with the seven prismatic colours, in the same purples, mixed with blue and yellow, other new colours manner as the last experiment. The colours for the may be composed.

orange, green, indigo, and violet, may be made by

mixing the other three. Then with bistre (FR) well III. Out of three of the primary colours, red, yellow, diluted, shade a sheet of very thin paper, by laying it

and blue, to produce all the oiher prismatic colours, light on both its sides. With pieces of this paper cover and all that are intermediate to them,

four.fifths of a glass, of the same size with the others,

by laying one piece on the four lowest divisions, anProvide three panes of glass (fig. 2.) of about five other on the three lowest, a third on the two lowest, inches square ; and divide each of them, by parallel and the fourth on the lowest division only, and leaving lines, into five equal parts. Take three sheets of very the top division quite uncovered. When one of the thin paper ; which you must paint lightly, one blue, coloured glasses is placed in the box, together with the another yellow, and the third red (D). Then paste on glass of shades, so that the side AB of the one be apone of the glasses five pieces of the red paper, one of plied to the side BC of the other, as in fig. 3. the sewhich must cover the whole glass, the second only the veral gradations of colours will appear shaded in the four lower divisions, the third the three lower, the same manner as a drapery judiciously painted with that fourth the two lowest, and the fifth the last division colour. only. On the other glasses five pieces of the blue and It is on this principle that certain French artists yellow papers must be pasted in like manner. You must have proceeded in their endeavours to imitate, by dealso have a box of about six inches long, and the same signs printed in colours, paintings in oil: which they depth and width as the glasses ; it must be black on do by four plates of the same size, on each of which is the inside : let one end be quite open, and in the engraved the same design. One of these contains all opposite end there must be a bole large enough to see the shades that are to be represented, and which are the glasses completely. It must also open at the top, painted either black or with a dark gray. One of the that the glasses may be placed in it conveniently. three other plates is covered with blue, another with When

you have put any one of these glasses in the red, and the third with yellow ; each of them being box, and the open end is turned toward the sun, you engraved on those parts only which are to represent will see five distinct shades of the colour it contains. that colour (G); and the engraving is either stronger If you place the blue and yellow glasses together, in a or weaker, in proportion to the tone of colour that is similar direction, you will see five shades of

green

dis- to be represented (H). tinctly formed. When the blue and red glasses are These four plates are then passed alternately under

the

Fig. 2.

:

(D) Water-colours must be used for this purpose : the blue may be that of Prussia, and very bright; the red, carmine; and the yellow, gamboge, mixed with a little saffron. These colours must be laid very light and even on both sides of the paper,

(2) In the first position of the glasses, the quantity of blue and yellow being equal, the same sort of green was constantly visible; but hy thus inverting the glasses, the quantity of the colours being constantly unequal, a very pleasing variety of tints is produced.

(F) The bistre here used must be made of soot, not that in stone.

(G) When a red drapery is required; it is engraved on the plate assigned to that colour; and so of yellow and blue : but if one of the other colours be wanting, suppose violet, it must be engraved on those that print the red blue; and so of the rest. The plates of this kind bave been hitherto engraved in the manner of mezzotinto ; but these, unless they are skilfully managed, are soon effaced. Engravings in the manner of crayon will perhaps answer better.

(H) Tae principal difficulty in this sort of engraving arises from want of skilful management, in giving each plate that precise degree of engraving which will produce the tone of colour required. If a bright green

Fig. 4.

the press, and the mixture of their colours produces a pear to be painted with the most lively colours in
print that bears no small resemblance to a painting. nature. If you cut on one of these papers the form of
It must be confessed, however, that what has been hi- the rainbow, about three quarters of an inch wide, you
therto done of this kind falls far short of that degree will have a lively representation of that in the atmo-
of perfection of which this art appears susceptible. If sphere.
they who engrave the best in the manner of the crayon,
were to apply themselves to this art, there is reason to
expect they would produce far more finished pieces than
we have hitherto seen.

Fig. 7.