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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ENGINEER AND MACHINIST’S DRAWING-BOOK.

tion of tliis face to the plane of projection, becomes at
that part of the surface situated nearest to the eye fainter
than the tint on the surface a b.

Surfaces in shade.—When a surface entirely in the
shade is parallel to the plane of projection, it should receive
a uniform dark tint.

When two objects parallel to each other are in the shade,
the one nearer the eye should receive the darker tint.

When a surface in the shade is inclined to the plane of
projection, those parts which are nearest to the eye should
receive the deepest tint.

The face b g h c, Fig. 1, PI. 56, projected horizontally
at b' g, Fig. 1, is situated in this manner. It will there
be seen that towards the line b c, the tint is much darker
than it is where it approaches the line g h.

If two surfaces exposed to the light, but unequally
inclined to its rays, have a shadow cast upon them, that
part of it which falls upon the surface more directly in-
fluenced by the light, should be darker than where it falls
upon the other surface.

Exemplifications of the foregoing rules may be seen on
various figures in the plates.

In order that these rules may be practised with proper
effect, we shall give some directions for using the brush,
or hair-pencil, and explain the usual methods employed
for tinting and shading.

The methods of shading most generally adopted are
either by the superposition of any number of flat tints,
or by tints softened off at their edges. The former method
is the more simple of the two, and should be the first
attempted.

Shading by fiat tints.—Let it be proposed to shade the
prism, Fig. 4, PL 56, by means of flat tints. According
to the position of the prism, as shown by its plan Fig. 1,
the face abed, Fig. 4, is parallel to the plane of projec-
tion, and, therefore, entirely in the light. This face should
receive a uniform tint either of Indian ink or sepia.
When the surface to be tinted happens to be very large,
it is advisable to put on a very light tint first, and then
to go over the surface a second time with a tint suffi-
ciently dark to give the desired tone to the surface.

The face, b g h c, being inclined to the plane of pro-
jection, as is shown by the line, b' g, in the plan, Fig. 1,
should receive a graduated tint from the line b c, to the
line g h. This graduality is obtained by laying on a
succession of flat tints in the following manner:—First,
divide the line, b' g, Fig. 1, into equal parts at the points,
1', 2', and from these points project lines upon, and parallel
to, the sides of the face, b g h c, Fig. 4. These lines
should be drawn very lightly in pencil, as they merely
serve to circumscribe the tints. A grayish tint is then
spread over that portion of the face, b g h c, Fig. 2, be-
tween the lines b c, and 1, 1. When this is dry, a similar
tint is to be laid on, extending over the space comprised
within the lines b c, and 2, 2, Fig. 3. Lastly, a third tint
covering the whole surface b c h g, Fig. 4, imparts the
desired graduated shade to that side of the prism. The
number of tints designed to express such a graduated

shade depends upon the size of the surface to be shaded;
and the depth of tint must vary according to this number.

As the number of these washes is increased, the whole
shade gradually presents a softer appearance, and the lines
which border the different tints become less harsh and
perceptible. For this reason the foregoing method of
representing a shade or graduated tint by washes succes-
sively passing over each other, is preferable to that some-
times employed, of first covering the whole surface, bghc,
with a faint tint, then putting on a second tint, 6 2 2 c;
followed, lastly, by a narrow wash, 6 1 1 c; because, in
following this process, the outline of each wash remains
untouched, and presents, unavoidably, a prominence and
harshness, which, by the former method, are in a great
measure subdued.

The face, a df e, is also inclined to the plane of pro-
jection, as shown by the line, a e, in the plan, Fig. 1 ;
but, as it is entirely in the light, it should be covered by
a series of much fainter tints than the surface, 6 g h c,
which is in the shade, darkening, however, towards the
line, e f. The gradation of tint is effected in the same
way as on the face, bghc.

Let it be proposed to shade a cylinder, Fig. 12, PI. 56,
by means of flat tints,

In shading a cylinder it will be necessary to consider
the difference in the tone proper to be maintained between
the part in the light and that in the shade. It should be
remembered that the line of separation between the light
and shade, a 6, is determined by the radius, O a, Fig. 5,
drawn at an angle of 45°, and perpendicular to the rays
of light. That part, therefore, of the cylinder, which is
in the shade, is comprised between the lines, a 6, and c d.
This portion, then, should be shaded conformably to the
rule previously laid down for treating surfaces in the shade
inclined to the plane of projection. All the remaining
part of the cylinder which is visible, presents itself to the
light; but, in consequence of its circular figure, the rays
of light form angles varying at every part of its surface,
and, consequently, this surface should receive a graduated
tint. In order to represent with effect the rotundity, it
will be necessary to determine with precision the part of
the surface which is most directly affected by the light.
This part, then, is situated about the line, e i, Fig. 12, in
the vertical plane of the ray of light, R 0, Fig. 5. As the
visual rays, however, are perpendicular to the vertical
plane, and, therefore, parallel to Y 0, it follows that the
part which appears clearest to the eye will be near this

line V 0, and may be limited by the line T 0, which

bisects the angle Y 0 R, and the line R 0. By pro-
jecting the points e and m, and drawing the lines e i,

and m n, Fig. 12, the surface comprised between these
lines will represent the lightest part of the cylinder.

This part should have no tint upon it whatever, if the
cylinder happen to be polished: a turned iron shaft, or a
marble column for instance ; but if the surface of the
cylinder be rough, as in the case of a cast-iron pipe, then
a very light tint—considerably lighter than on any other
part—may be given it.
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