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 proceed from the image of the object from which the light originally fell on the mirror.

37. There is a distinction, though not a very important one, between some optical images and others. In most cases in which the reflexion is made by a concave mirror, the rays proceeding from the various points of an object are made to converge, and actually meet in points constituting the image, and afterwards diverge from those points, as they would from a real surface placed there, so as to make it visible. In other cases the reflexion only changes the foci from which the rays diverge, and it is only by being produced backwards, that the rays can be said to meet in certain points.

38. There are some cases in which an optical image does in fact present the appearance of something real. If a candle be held before a concave mirror in a dark room, there will often be seen an appearance of an inverted flame in the air, owing to the reflected light converging to foci in front of the mirror, and there illuminating strongly the particles of dust floating about in the room. The image may generally be made more evident by holding a piece of paper at the place to be illuminated by the reflected light.

39. In the case of a plane mirror we have seen, that when any pencil of rays falls on it, the reflected light proceeds from a point at the same distance behind the mirror, as the focus of incident rays is before it. It will therefore be easily seen, (Fig. 34.) that the image of an object placed before such a mirror must be exactly similar to the object itself in point of form, all the points in the surface of the object having points corresponding to them which are similarly situated with respect to each other. Thus when a man looks at his reflexion in a plane mirror or common looking-glass, he sees a figure exactly like himself in form and dimensions, only that what is the right hand of the figure answers to his left, and vice-versâ. This reversion of the figure of course takes place in all such reflexions.