object, we are then able to distinguish them from each other, by their relative position, also.

4. How do you illustrate the idea of form? If we join any three points A, B, and C, by straight lines, the result will be a figure or form of a triangle. If we take another point D, and join the three points A, D, and C, we shall have the form of another triangle ADC. The straight lines which bound each of these figures, make up what is called its out

line.

If with C as a centre, and any radius, we describe the circumference of a circle, the curve

so drawn will be the outline of the circle.

C

Now, the triangle and circle differ from each other only in form, and the form is determined by the outline: hence we see that outline is one means of representing form to the eye. It is thus that we are able to distinguish a triangle from a circle, and a circle from a square; and the drawings of their outlines present to the mind, through the eye, the idea of the objects themselves.

5. How do you illustrate light and shade?

If we hold any object in the sun's rays, it is evident, that that part of it which is turned towards the sun will be lighted; and that the part which is turned away from the

sun will be comparatively dark. The part towards the sun is called the light; the other part, the shade.

6. In what manner do light and shade modify the idea of a form which is represented only by its outline?

The circle whose centre is C,

is the outline of so much of the flat white paper as is contained within its circumference.

Now, if we observe a sphere, or perfectly round ball, we find that, in every position, its outline is also a circle. We cannot tell, therefore, whether this circle is the out

C

line of a circular piece of paper or of a sphere.

Let the circle whose centre is D, be the outline of a sphere. If we suppose the light to proceed from the left hand, then the part of the sphere towards the left will be the light, and the part towards the right, the shade.

Leaving the white paper for the light, we will represent the shade, or dark part, by means of lines drawn in such a manner, as to darken that part occupied by the shade.

D

7. In how many ways may the shade on a body be modi fied?

In two ways: viz, in its depth or intensity, and its color.

8. How do you know which part of a body has the greatest depth or intensity of shade?

If there were no atmosphere, and no body in existence except the one we are considering, that part of it which does not receive the sun's rays would be invisible. But since the atmosphere, as well as every other substance in nature, reflects back the light which it receives, casting it in a direction contrary to that of the sun's rays; it follows, that the part of any object which does not receive the direct light of the sun, will yet receive light from other objects, behind it with reference to the sun, and will be sufficiently illuminated to exhibit its form. Now, since bodies are more or less illuminated as they receive the light directly or obliquely, it follows, that if we conceive the reflecting body to be placed directly behind the one receiving the light, that the part nearest the reflecting body will receive more light than the parts more remote; and hence, the shade there will be less intense. It therefore follows, that the effect of reflected light on the depth of shade, will be the greatest near the outline of the body which is farthest from the source of light.

1

9. How may the shade of an object be modified in regard to color?

Every reflected ray of light is of the same color as the body which reflects it, and when such rays illuminate a dark object, they also impart to it their color. This may be shown by holding any dark body, as a sphere, in the sun's rays, and placing near it, and opposite to the sun, a piece of bright-colored red or yellow paper. The reflected rays from the paper will impart their tint to the shade of the sphere.

10. What is the shadow on a body?

The shadow on a body is that part of it from which the light is intercepted by some opaque body.

11. How may the forms of objects be discovered by means of the shadows which they cast or receive?

It is evident that the shadow of a triangle, or of a square, on a flat surface, will, in certain positions, exactly resemble the bodies which cast them. But the surface which receives the shadow will modify the shape of it; and thus the shadow will also give an idea of the form of the surface on which it falls.

For example, the rectangle in the figure casts a shadow of such a shape on the wall and step which are behind it, as to show their form distinctly. Without the shadow, the two lines which are the outlines of the step, might equally well represent two ho

rizontal lines drawn upon the wall.

12. How may the relative position of objects be determined by the shadows which they cast or receive?

When a shadow is entirely separated from the body which casts it, as is the shadow of the sphere in this example, it is then plain that a space intervenes between the body and the surface on which the shadow falls.

But when the shadow joins the body which casts it, as in this example, then the body

casting the shadow touches the surface on which the shadow falls.

Hence, the nearer an object is to the surface on which the shad

ow falls, the nearer will the shadow approach to the object.