know the properties of the convex

lens..

A convex lens placed in a hole of a window-shutter, will exhibit, on a white sheet of paper placed in the focus of the glass, all the objects on the outside, as fields, trees, men, houses, &c. in an inverted order.

James. Is the room to be quite dark, except the light which is admitted through the lens?

Tutor. It ought to be so; and, to have a very interesting picture, the sun should shine upon the objects.

James. Is there no other kind of camera obscura?

Tutor. A portable one may be made with a square box, in one side of which is to be fixed a tube, having

a convex lens in it: within the box! is a plane mirror reclining backwards. from the tube, in an angle of forty-fivei

Charles. On what does this mirror reflect the image of the object?

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Tutor. The top of the box is: a square of unpolished glass, on; which the picture is formed. And if a piece of oiled paper be stretched on the glass, a landscape may be easily copied; or the outline may be sketched on the rough surface of the glass.

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James. Why is the mirror to be placed at an angle of 45 degrees exactly?

Tutor. The image of the objects would naturally be formed at the

back of the box opposite to the lens; in order, therefore, to throw it on the top, the mirror must be so placed that the angle of incidence shall be equal to the angle of reflection. In the box, according to its original make, the top is at right angles to the end, that is, at an angle of 90 degrees, therefore the mirror is put at half 90, or 45 degrees.

Charles. Now the incident rays falling upon a surface which declines to an angle of 45 degrees, will be reflected at an equal angle of 45 degrees, which is the angle that the glass top of the box bears with respect to the mirror.

James. If I understand

you clear

ly, had the mirror been placed at

the end of the box, or parallel to it, the rays would have been reflected back to the lens; and none would have proceeded to the top of the box.

Tutor. True: in the same manner as when one person stands before a looking-glass, another at the side of the room cannot see his image in the glass, because the rays flowing from him to the looking-glass are thrown back to himself again; but let each person stand on the opposite side of the room, while the glass is in the middle of the end of it, they will both stand at an angle of 45 degrees, with regard to the glass, and the rays from each will be reflected to the other.

Charles. Is the tube fixed in this

machine?....

4

Tutor. No; it is made to draw out, or push in, so as to adjust the distance of the convex glass from -the mirror, in proportion to the distance of the outward objects, till they are distinctly painted on the horizontal glass.

James. Will you now explain the structure of the magic-lanthorn, which has long afforded us occasional amusement?.

Tutor. This little machine consists, as you know, of a sort of tin box; within which is a lamp or candle, the light of this passes through a great plano-convex lens, placed in a tube fixed in the front.