figure only, which being painted in transparent colours, the light shines through it, and no light can come upon the screen but what passes through the figure.

James. But there was no sheet to receive the picture.

Tutor. No: the representation was thrown on a thin screen of silk placed between the spectators and the lantern.

Charles. What caused the images to appear approaching and receding?

Tutor. It is owing to removing the lantern farther from the screen, or bringing it nearer to it; for, the size of the image must increase, as the lantern is carried back, because the rays come in the shape of a cone, and as no part of the screen is visible, the figure appears to be formed in the air, and to move farther off when it becomes smaller, and to come nearer as it increases in size.

James. Here is another instrument, the construction of which you promised to explain: the multiplying glass.

Tutor. One side of this glass is cut into many distinct surfaces, and in looking at an object, as your brother, through it, you will see not one object only, but as many as the glass contains plane surfaces.

I will draw a figure to illustrate this: let (Plate vi. Fig 42.) A i B represent a glass, flat at the side next the eye H, and cut into three

distinct surfaces on the opposite side, as ab,bd, d B. The object c will not appear magnified, but as rays will flow from it to all parts of the glass, and each plane surface will refract these rays to the eye, the same object will appear to the eye in the direction of the rays, which enter it through each surface. Thus a ray ci falling perpendicularly on the middle surface, will suffer no refraction, but show the object in its true place at c: the ray from c b, falling obliquely on the plane surface a b, will be refracted in the direction be, and on leaving the glass at e, it will pass to the eye in the direction e H, and therefore it appears at E; and the ray c d will, for the same reason, be refracted to the eye in the direction B H, and the object c will appear also in D.

If, instead of three sides, the glass has been cut into 6, or 20, or any other number, there would have appeared 6, 20, &c. different objects differently situated.

MAGNETISM.

L 2

CONVERSATIÓN XXIII.

Of the Magnet: its Properties: useful to Mariners, and others; Iron rendered Magnetic; Properties of the Magnet.

TUTOR. You see this dark brown mineral body, it is almost black, and you know it has the property of attracting needles and other small iron substances.

James. Yes, it is called a load-stone, leadingstone, or magnet; we have often been amused with it but you told us that it possessed a much more important property than that of attracting iron and steel.

Tutor. This is what is called the directive property, by which mariners are enabled to conduct their vessels through the mighty ocean, out of the sight of land: by the aid of this, miners are guided in their subterranean inquiries, and the traveller through deserts, otherwise impassable.

Charles. Were not mariners unable to make long and very distant voyages till this property of the magnet was discovered?