Armengaud, Jacques Eugène; Leblanc, César Nicolas   [Hrsg.]; Armengaud, Jacques Eugène   [Hrsg.]; Armengaud, Charles   [Hrsg.]
The engineer and machinist's drawing-book: a complete course of instruction for the practical engineer: comprising linear drawing - projections - eccentric curves - the various forms of gearing - reciprocating machinery - sketching and drawing from the machine - projection of shadows - tinting and colouring - and perspective. Illustrated by numerous engravings on wood and steel. Including select details, and complete machines. Forming a progressive series of lessons in drawing, and examples of approved construction — Glasgow, 1855

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MANIPULATION OF SHADING AND SHADOWS.

89

rectly projected, their direction is an arbitrary matter,
and may be left to the taste of the draughtsman.

Figs. 2, 3, and 6, exemplify the complex appearance of
shade and shadow presented on concave surfaces. It is
worthy of particular notice, that the shadow on a concave
surface is darkest towards its outline, and becomes lighter
as it nears the edge of the object. Reflection from that
part of the surface on which the light falls most power-
fully, causes this gradual diminution in the depth of the
shadow; the greatest amount of reflection being opposite
the greatest amount of light.

It may be as well to remark here that no brilliant or
extreme lights should be left on concave surfaces, as such
lights would tend to render it doubtful, at first sight,
whether the objects represented were concave or convex.
After the body-colour—which shall be treated in a subse-
quent section—has been put on, a faint wash should be
passed very lightly over the whole concavity. This will
not only modify and subdue the light, but tend to soften
any asperities in the tinting, which are more unsightly on
a concave surface than on any other.

The lightest part of a sphere (Fig. I) is confined to a
mere point, around which the shade commences and
gradually increases as it recedes. This point is not in-
dicated on the.figure referred to, because the shade tint
on a sphere ought not to be spread over a greater portion
of its surface than is shown there. The very delicate
and hardly perceptible progression of the shade in the
immediate vicinity of the light point, should be effected
by means of the body-colour of the sphere. If, for in-
stance, the material of which the sphere is composed be
brass, the body-colour itself should be lightened as it nears
the light point. In like manner, all polished or light-
coloured curved surfaces should be treated;—the part bor-
dering upon the extreme light being covered with a tint
of body-colour somewhat fainter than that used for the
flat surfaces. Again, if the sphere be of cast-iron, then
the ordinary body-colour should be deepened from the light
point until it meets the shade tint, over which it is to be
spread uniformly. Any curved unpolished surface is to
be thus treated;—the body-colour should be gradually
deepened as it recedes from that part of the surface most
exposed to the light. Considerable management is neces-
sary in order to shade a sphere effectively. The best way
is to put on two or three softened-off tints in the form of
crescents converging towards the light point, the first one
being carried over the point of deepest shade.

A ring (Fig. 5) is a difficult object to shade. To change
with accurate and effective gradation the shade from the
inside to the outside of the ring, to leave with regularity
a line of light upon its surface, and to project its shadow
with precision, require a degree of attention and care in
their execution, greater perhaps than the shade and shadow
of any other simple figure. The learner, therefore, should
practise the shading of this figure, as he will seldom meet
with one presenting greater difficulties.

Figs. 7 and 8, show the peculiarities of the shadows
cast by a conical form on a sphere or cylinder. The fol-

lowing fact should be well noted in the memory:—That
the depth of a shadow on any object is in proportion to
the degree of light which it encounters on the surface ot
that object. In these figures very apt illustrations of this
fact may be remarked. It will be seen, by referring to
the plan, Fig. 7, that the shadow of the apex of the cone
happens to fall upon the lightest point of the sphere, and
is, therefore, the darkest part of the shadow. So also the
deepest portion of the shadow of the cone on the cylinder,
in the plan, Fig. 8, is exactly where it coincides with the
line of extreme light. Flat surfaces are similarly affected;
the shadows thrown on them being less darkly expressed,
according as their inclination to the plane of projection
increases. The body-colour on a flat surface should, on the
contrary, increase in depth as the surface becomes more
inclined to this plane.

Another notable fact is exemplified by these figures: —
that reflected light is incident to shadows as well as to
shades. This is very observable where the shadow of the
cone falls upon the cylinder. It may likewise be remarked,
though to a less extent, on other parts of these figures.
The reflected light on the cone from the sphere or cylinder
is also worthy of observation. This light adds greatly to
the effect of the shadows, and indeed to the appearance
of the objects themselves. Altogether, these figures offer
admirable scope for study and practice.

The concentration, within a small space, of nearly all
the peculiarities and effects of light, shade, and shadow,
may be seen on Plate 58, in the examples of screws there
given.

The parts of a highly-finished coloured drawing of a
machine are always affected by a certain degree of inde-
finableness in their outline.

Notwithstanding the most careful exertions of the
colourist to keep every feature of a machine clear and dis-
tinct, some amount of uncertainty, resulting unavoidably
from the proximity and natural blending of the different
parts, will pervade the lines which separate its component
members. For practical working purposes, therefore, a
completely coloured drawing of a machine is unsuitable.
On the other hand, a mere outline, although perhaps in-
telligible enough to those who are familiarly acquainted
with the machine delineated, has an undecided appear-
ance. As complete colouring renders it difficult for the eye
to separate the various parts of a machine, owing to an
apparently too intimate relationship between them; a line
drawing, on the contrary, perplexes the eye to discover
any relation between them at all, or to settle promptly
their configuration. The eye involuntarily asks the ques-
tion, is that part round or square, or is it even a distinct
part of the machine at all ? As a means of avoiding the
indefiniteness presented by the outline in the former case,
and the want of adequate coherence and doubtfulness in
the form of the different parts amenable to the latter,
recourse is not unfrequentlyhadto a kind of semi-colouring,
or rather mere shading of the parts of a machine. An
exemplification of this style of representing the entire
views of a machine, and the various details of which it is
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