James. Is the image formed by a concave mirror always before it ?

Tutor. In all cases, except when the object is nearer to the mirror than the principal focus.

Charles. Is the image then behind the mirror?

Tutor. It is; and farther behind the mirror than the object is before it. Let A c (Plate 111. Fig. 18.) be á mirror, and a z the object between the centre K of the glass, and the glass itself; and the image r y z will be behind the glass erect, curved, and magnified, and of course the image is farther behind the glass than the object is before it.

James. What would be the effect if, instead of an opaque object rx z, á luminous one, as a candle, were placed

in the focus of a concave mirror?

Tutor. It would strongly illuminate a space of the same dimension as the mirror to a great distance; and if the candle were still nearer the mirror than the focus, its rays will enlighten a larger space. Hence you may understand the construction of many of the lamps which are now to be seen in many parts of London, and which are undoubtedly a great improvement in lighting the streets.

CONVERSATION XIII.

Of Concave and Convex Mirrors.

TUTOR. We shall devote another morning or two to the subject of reflection from mirrors of different kinds.

Charles. You have not said any thing about conver mirrors, and yet they are now very much in fashion in handsome drawing-rooms: I have seen several, and always observed that the image was very much less than the object.

Tutor. A convex mirror is an ornamental piece of furniture, especially if it can be placed before a window, either, with a good prospect, or where there are a number of persons passing and repassing in their different employments. The images reflected from these are smaller than the objects, erect, and behind the surface, therefore a landscape or a busy scene delineated on one of them, is always a beautiful object to the eye. For the same reason a glass of this kind in a room in which large assemblies meet, forms an extremely interesting picture. You may easily conceive how the convex mirror diminishes objects, or the images of objects, by considering in what manner they are magnified by the concave mirror. If y x (Fig. 18.) were

a straight object before a conver mirror A C, the image by reflection would

be r z.

James. Would it not appear curved?

Tutor. Certainly for if the object be a right line, or a plain surface, its image must be curved, because the different points of the object are not equally distant from the reflector. In fact, the images formed by convex mirrors, if accurately compared with the objects, are never exactly of the same shape.

Charles. I do not quite comprehend in what manner reflection takes place at a convex mirror.

Tutor. I will endeavour by a figure to make it plain: C D (Plate III. Fig. 19.) represents a convex mirror standing at the end of a room, before which the arrow A B is placed on one