n, as seen behind the mirror; but by reflection they are made to diverge less than before, and consequently to make the angle under which they meet more obtuse at the eye b, than it would be if they continued onward to e, where they would have met without reflection. The result, therefore, is to render the image h, upon the eye, as much larger than the object a, as the angle at the eye is more obtuse than the angle at e.

On the contrary, if the object is placed more remote from the mirror than the principal focus, and between the focus and the centre of the sphere of which the reflector is a part, then the image will appear inverted on the contrary side of the cenFig. 146.

tre, and farther from the mirror than the object; thus, if a lamp be placed obliquely before a concave mirror, as in fig. 146, its image will be seen in

verted in the air on the contrary side of a perpendicular line through the centre of the mirror.

From the property of the concave mirror to form an inverted image of the object suspended in the air, many curious and surprising deceptions may be produced. Thus, when the mirror, the object, and the light, are placed so that they cannot be scen, (which may be done by placing a screen before the light, and permitting the reflected rays to pass through a small aperture in another screen,) the person mistakes the image of the object for its reality, and not understanding the deception. thinks he sees persons walking with their heads downwards and cups of water turned bottom upwards without spilling a drop. Again, he sees clusters of delicious fruit, and when invited to help himself, on reaching out his hand for that purpose, he finds that the object either suddenly vanishes from his sight, owing to his having moved his eye out of the proper range, or that it is intangible.

This kind of deception may be illustrated by any one who has a concave mirror only of three or four inches in diameter, in the following manner:

When will the image from the concave mirror be inverted and before the mirror? What property has the concave mirror by which singular deceptions may be produced? What are these deceptions?

Suppose the tumbler a, to be filled with water, and placed beyond the principal focus of the concave mirror, fig. 147, and so managed as to be hid from the eye c, by the screen b. The lamp by which the tumbler is illuminated must also be placed behind the screen, and near the tumbler. To a person placed at c, the tumbler with its contents will appear inverted at e, and suspended in the air. By carefully moving forward, and still keeping the eye in the same line with respect to the mirror, the person may pass his hand through the shadow of the Fig. 147.

tumbler; but without such conviction, any one unacquainted with such things, could hardly be made to believe that the image was not a reality.

By placing another screen between the mirror and the image, and permitting the converging rays to pass through an aperture in it, the mirror may be nearly covered from the eye, and thus the deception would be increased.

The image reflected from a concave mirror, moves in the same direction with the object, when the object is within the principal focus; but when the object is more remote than the principal focus, the image moves in a contrary direction from the object, because the rays then cross each other. If a man place himself directly before a large concave mirror, but farther from it than the centre of concavity, he will see an inverted image of himself in the air, between him and the mirror, but less than himself. And if he hold out his hand towards the mirror, the hand of his image will come out toward his hand, and he may imagine that he can shake hands

Describe the manner in which a tumbler with its contents may be made to seem inverted in the air. Why does the image move in a contrary direction from its object, when the object is beyond the principal focus ?

with his image. But if he reach his hand further towards the mirror, the hand of the image will pass by his hand, and come between his hand and his body; and if he move his hand toward either side, the hand of the image will move in a contrary direction, so that if the object moves one way the image will move the other.

The concave mirror, having the property of converging the rays of light, is equally efficient in concentrating the rays of heat, either separately, or with the light. When, therefore, such a mirror is presented to the rays of the sun, it brings them to a focus, so as to produce degrees of heat in proportion to the extent and perfection of its reflecting surface. A metallic mirror of this kind, of only four or six inches in diameter, will fuse metals, set wood on fire, &c.

As the parallel rays of heat or light are brought to a focus at the distance of one quarter of the diameter of the sphere, of which the reflector is a section, so if a luminous or heated body be placed at this point, the rays from such body passing to the mirror will be reflected from all parts of its surface, in parallel lines; and the rays so reflected, by the same law, will be brought to a focus by another mirror standing opposite to this Fig. 148. 1

b

Suppose a red hot ball to be placed in the principal focu of the mirror a, fig. 148, the rays of heat and light proceed ing from it will be reflected in the parallel lines 1, 2, 3, &c. The reason of this is the same as that which causes parallel rays, when falling on the mirror, to be converged to a

Will the concave mirror concentrate the rays of heat, as well as those of light? Suppose a luminous body be placed in the focus of a concave mirror, in what direction will its rays be reflected?

focus. The angles of incidence being equal to those of reflection, it makes no difference in this respect, whether the rays pass to or from the focus. In one case, parallel incident rays from the sun, are concentrated by reflection; and in the other, incident diverging rays, from the heated ball, are made paral lel by reflection.

The rays therefore, flowing from the hot ball to the mirror a, are thrown into parallel lines by reflection, and these reflected rays, in respect to the mirror b, become the rays of incidence, which are again brought to a focus by reflection.

Thus the heat of the ball, by being placed in the focus of one mirror, is brought to a focus by the reflection of the other mirror.

Several striking experiments may be made with a pair of concave mirrors placed facing each other as in the figure. If a red hot ball be placed in the focus of a, and some gun powder in the focus of b, the mirrors being ten or twenty feet apart, according to their dimensions, the powder will flash by the heat of the ball, concentrated by the second mirror. To show that it is not the direct heat of the ball which sets fire to the powder, a paper screen may be placed between the mirrors antil every thing is ready. The operator will then only have to remove the screen, in order to flash the powder.

To show that heat and light are separate principles, place a piece of phosphorus in the focus of b, and when the ball is so cool as not to be luminous, remove the screen, and the phosphorus will instantly inflame.

Refraction by Lenses.

A Lens is a transparent body, generally made of glass, and in shaped that the rays of light in passing through it are either collected together or dispersed. Lens is a Latin word, which comes from lentile, a small flat bean.

It has already been shown, that when the rays of light pass from a rarer to a denser medium, they are refracted, or bent out of their former course, except when they happen to fall perpendicularly on the surface of the medium.

The point where no refraction is produced on perpendicular rays, is called the axis of the lens which is a right line

Explain fig. 148, and show why the rays from the focus of a are concentrated in the focus b. What curious experiments may be made by two concave mirrors placed opposite to each other? How may it be shown that heat and light are distinct principles ? What is a lens?

passing through its centre, and perpendicular to both its sur faces.

In every beam of light, the middle ray is called its axis. Rays of light are said to fall directly upon a lens, when their axes coincide with the axes of the lens; otherwise they are said to fall obliquely.

The point at which the rays of the sun are collected, by passing through a lens, is called the principal focus of that lens. Lenses are of various kinds, and have received certain names, depending on their shapes. The different kinds are shown at fig. 149.

Fig. 149.
a

P

A prism, seen at a, has two plane surfaces, a r, and a s, in clined to each other.

A plane glass, shown at b, has two plane surfaces, parallel to each other.

A spherical lens, c, is a ball of glass, and has every part of 'ts surface at an equal distance from the centre.

A double concave lens, d, is bounded by two convex surfaces opposite to each other.

A plano-concave lens, e, is bounded by a convex surface on one side, and a plane on the other.

A double-concave lens, f, is bounded by two concave spherical surfaces, opposite to each other.

A plano-concave lens, g, is bounded by a plane surface on one side and a concave one on the other.

A meniscus, h, is bounded by one concave and one convex spherical surface, which two-surfaces meet at the edge of the lens.

A concavo-convex lens i, is bounded by a concave and convex surface, but which diverge from each other, if continued,

What is the axis of a lens? In what part of a lens is no refraction produced? Where is the axis of a beam of light? When are rays of light said to fall directly upon a lens? How many kinds of lenses are mentioned? What is the name of each? How are each of these lenses bounded?