Emily. There is no image formed of me in the glass now. Mrs. B. I beg your pardon, there is; but you cannot see it, because the incident rays falling obliquely on the mirror will be reflected obliquely in the opposite direction, the angels of incidence and of reflection being equal. Caroline, place yourself in the direction of the reflected rays, and tell me whether you do not see Emily's image in the glass? Caroline. Let me consider. In order to look in the direction of the reflected rays, I must place myself as much to the left of the glass as Emily stands to the right of it. Now I see her image but it is not straight before me, but before her; and appears at the same distance behind the glass, as she is in front of it. Mrs. B. You must recollect, that we always see objects in the direction of the last rays which reach our eyes. Figure 4. represents an eye looking at the image of a vase reflected by a mirror; it must see it in the direction of the ray A B, as that is the ray which brings the image to the eye: prolong the ray to C, and in that spot will the image appear. Caroline. I do not understand why a looking-glass reflects the rays of light: for glass is a transparent body which should transmit them ? Mrs. B. It is not the glass that reflects the rays which form the image you behold, but the mercury behind it. The glass acts chiefly as a transparent case, through which the rays find an easy passage. Caroine. Why then should not mirrors be made simply of mercury? Mrs. B. Because mercury is a fluid. By amalga mating it with tin-foil, it becomes of the consistence of paste, attaches itself to the glass, and forms in fact a mercurial mirror, which would be much more perfect without its glass cover, for the purest glass is never perfectly transparent; some of the rays therefore are lost during their passage through it, by being either absorbed, or irregularly reflected. This imperfection of glass mirrors has introduced the use of metallic mirrors, for optical purposes. Emily. But since all opaque bodies reflect the rays of light, I do not understand why they are not all mirrors? Caroline. A curious idea indeed, sister; it would be very gratifying to see one's self in every object at which one looked. Mrs. B. It is very true that all opaque objects reflect light; but the surface of bodies in general is so rough and uneven, that their reflection is extremely irregular, which prevents the rays from forming an image on the retina. This you will be able to understand better, when I shall explain to you the nature of vision, and the structure of the eye. You may easily conceive the variety of directions in which rays would be reflected by a nutmeg-grater, on account of the inequality of its surface, and the number of holes with which it is pierced. All solid bodies resemble the nutmeg-grater in these respects, more or less; and it is only those which are susceptible of receiving a polish, that can be made to reflect the rays. with regularity. As hard bodies are of the closest texture, the least porous, and capable of taking the highest polish, they make the best mirrors; none therefore are so well calculated for this purpose as metals. Caroline. But the property of regular reflection is not confined to this class of bodies; for I have often seen myself in a highly polished mahogany table. Mrs. B. Certainly; but as that substance is less durable, and its reflection less perfect, than that of metals, I believe it would seldom be chosen for the pupose of a mirror. There are three kinds of mirrors used in optics; the plain or flat, which are the common mirrors we have just mentioned; convex mirrors; and concave mirrors. The reflection of the two latter is very different from that of the former. The plain mirror, we have seen, does not alter the direction of the reflected rays, and forms an image behind the glass exactly similar to the object before it. A convex mirror has the peculiar property of making the reflected rays diverge, by which means it diminishes the image; and a concave mirror makes the rays converge, and, under certain circumstances, magnifies the image. Emily. We have a convex mirror in the drawingroom, which forms a beautiful miniature picture of the objects in the room; and I have often amused myself with looking at my magnified face in a concave mirror. But I hope you will explain to us why the one enlarges while the other diminishes the objects it reflects. Mrs. B. Let us begin by examining the reflection of a convex mirror. This is formed of a portion of the exterior surface of a sphere. When several parallel rays fall upon it, that ray only which, if prolonged, would pass through the centre or axis of the mirror, |