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when one person stands before a looking glass, another at the side of the room canno 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 rays from each will be reflected to the other.
Charles. Is the tube fixed in this machine?
Tutor. 1o; it is made to draw out, or push in, so as to adjust the distance of the convex glass from the mirror, in proportioa 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. This strongly illuminates the objects which are painted on slips of glass, and placed before the lens in an inverted position. A sheet, or other white surface, is placed to receive the images.
Charles. Do you invert the glasses on which the figures are drawn, in order that the images of them may be erect.
Tutor. Yes: and the illumination may be greatly increased, and the effect much more powerful, by placing a concave mirror at the back of the lamp.
Charles. Did you not tell us that the Phantasmagoria, which we saw at the Lyceum, was a species of the magic lanthorn?
Tutor, There is this difference between them : in common magic lanthorns, the figures are painted on transparent glass, consequently the image on the screen is a circle of light, having a figure or figures on it; but in the Phantasmagoria, all the glass is made opaque, except the 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 lanthorn.
Charles. What caused the images to appear approaching and receding?
Tutar. It is owing to removing the lan. thorn farther from the screen, or bringing it nearer to it; for, the size of the image must increase, as the lanthorn is carried back, because the rays come in the shape of a cone, and as no part of the screen is visi. ble, 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 ex.. plain : 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 #, and cut into three distinct surfaces on the opposite side, as a b, b d, d B. The object c will not appear imagnified, but as rays will flow from it to all parts of the glass, and each plane surface will refract these rays tu 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 o i falling perpendicularly on the middle surface, will suffer no refraction, but show the object in its true place at c: the ray from c 6 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 1, and the object c will appear also