IN the issue of December 24th, 1910, of Scientific AmeRican Mr. Herring propounded the problem: “Why does a mirror reverse right and left, but not up and down?” In an answer by Mr. Keen (January 28th, 1911) it was rightly pointed out that the cause of this inversion of printed or written characters (the example cited by Mr. Herring) lies in the manner of presenting the same to the mirror. It is the observer who turns the sheet about, and not the mirror. Now at first sight it may appear that this explanation completely settles the whole question, and that the image in the mirror is indeed a true and exact copy o'f the original. In point of fact this is not in general the case, although it is in a sense true of an object having only two dimensions, such as the plane surface of a sheet of paper, and the writing upon it. For this reason, in order to obtain a full understanding of what the mirror does, it is necessary to choose another object for our example, one which extends in the three dimensions of space. Let us take first of all a simple case, which for our immediate purpose may be still two-dimensional, provided it does not lie in a plane parallel to that of the mir101. Suppose we prfsent to the mirror a V-shaped object ODE, for example, a wire bent to the shape indicated (Figs. 1 and 2). To an observer in front of the mirror, the image of ODE will appear located at 0' D' E', as shown in perspective in Fig. 1 and in plan in Fig. 2. Note that the point D which lies farthest back in the original object, in the image is represented by the point IY lying farthest forward. Note also that 0' D' E' may be produced from ODE by turning the latter iJide out, an( displaCing it a certain distance backward. If this is not immediately obvious, imagine ODE to be made of flexible material, such as string. If the point D is held fast, and 0 and E are pulled forward through a certain distance, evidently a figure like 0' D' E' is obtained. We may therefore say that what the mirror does to ODE is to turn it inside out. Now, so long as the object is merely two· dimensional, as in the present example, there is little gained by this point of view. For we might with equal justice say that ODE has simply been rotated about AB through 180 degrees. The matter assumes a very different aspect as soon as we turn to a three-dimensional example. Suppose a glove is lying on a table in front of the mirror AB, as indicated in our second illustration (Figs. 3 and 4). What will its image be? Suppose the glove was that which, fts your right hand; the image will be such a glove as you would obtain by t urn i n g your right hand glove inside out-a left glove. Figs. 3 and 4 illustrate this case. If you lre not conv inc ed, try it. Suppose a one-legged man could be put through the 0 perat ion which the mirror performs u p 0 n you. Should he have had the ill luck to take off his shoe before he u n del went the change, so that the shoe retained its original character, he would now find that it no longer fitted his only foot! For if this was his right originally, it is now his left, and his shoe no longer fts it (Fig. 5). He might of course easily avoid this awkward predicament by wearing his shoe whiJe being “turned inside out,” in which case his shoe also would be converted into a left shoe. The perfectly symmetrical man would not be affected in any way, as regard: (Continued on page 197.) What the Mirror Does to You (Continued from 190.) his clothing, only he would have to put his right glove on what used to left hand ! While there is a certain comic element in the point of view thus presented, i t has as a matter of fact a number of important serious applications. The first point to note is that no ordinary operation, such as mere rotation or inversion, can possibly effect the change which the mirror produces. No turning or twisting of your right hand glove can ever make it into a left hand glove-except that one operation of "turning it inside out." With most ordinary objects this operation is o f course quite impossi ble. Two ordinary o bjects therefore which are related to one another as the right hand is to the left, Or in other words, as one object is to its mirror image, cannot by any sort of transposition be made t o coincide, like two congruent figures of geometry. This fact has found a n interesting and important application in organic chemistry. It is found that 'certain chemical compounds occur in pairs, the two members of each pair being practically identical in all their properties, but while on e rotates the plane of polarization of polarized light to the right, the other rotates the same by an equal amoun t to the left . Now the curious thing is observed, that in every case in which the formula of such a compound is known, it is found to contain a so-called asymmetric carbon atom, that is to say, one which is linked to 'four different varyor ra dicles. An example of th is lactic acid, whose formula is CH. It will be observed that the central carbon - atom has four unlike groups attached to it. If a solid model of such a formula is made, with the linl{s extending evenly out from the central carbon atom, it is found that there are two possible arrangements of the groups-and the two are related to each other as an object is to its mirror image, or as the left hand is to the right. This is shown in our last illustration (Figs. 6 and 7) , in which the central carbon atom of the lactic acid molecule is represented in the customary manner by a regula r tetrahedron, or four-angled solid. Each corner of this tetrahedron represents one of the bonds or affinities of th e central ca rbon atom. The four groups attached can be arranged in two and only two insuperposable arrangements, as shown, of which the one is the mirror image of the other. This is true on ly as long as the groups attached to the central carbon atoms are all dissimilar, as is the case in lactic acid. As soon as two or more of them are alike, only one arrangement is possible. The fact that such an " asymmetric" carbon atom occurs in all optically active carbon compounds whose formula is known (and they are exceed- in gly numerous) appears too significant to be ascribable to accident, and it is customary to regard the t wo twin 70 50 .0 106 .7 170 .0 formsoptical isomers a s they are called -as represented by the two possible space formulre, although there is at present no means of identifying each member of the pair of compounds with a particular one of these two formulre. The reader will recall that in analytical geometry also it is necessary to distinguish between two kinds of systems lated to one another as object to im age, I Science. viz:- or as the right hand is to the left hand. ROYAL COLLEGE OF SCIENCE. Here also the two arrangements are l','I athematic8 and Mechanics' Prolessor PERRY. F. R. s. "insuperposable," and form tw o d ist inct Physico-'Prolessor CALLENDAR. F. R . s . systems. Since mathematical treatment Professor The Hon. R. J. ST RUTT. F R. s. exten ds to innumerable applications of Chemistry (including Chemical Technology) all kinds in different branches of science, this relation of the object to its image is one of pretty general interest. A New “ Vanishing' Trick “ Box A FAVORITE trick both with the professional conjurer and the amateur is to cause the unaccountable appearance seemingly from, nowhere of various objects or persons, and Itheir equally mysterious disappearance. Various artifices and devices have been used, according to circumstances, to produce these enigmatic effects. Our illustration shows a new form of trick box, Iby the aid of which the conjurer may for example exhibit the remarkable manifestation of “the vanishing lady.” The mechanism of the device is very simple. The Ibox is provided with a drawer, which slides out backward, in the direction away from the audience, so that when withdrawn it is hidden from view, its front panel then forming the back wall of the !box as seen by the aUdience, when the flaps in the sides of the box are opened for inspection. The conjurer starts by opening the lid at the top of the box. A system of cords (IF) and pulleys is so arranged that this action of lifting the cover at the same time causes the drawer, containing the vanishing lady, to retreat blkward into the position shown in the body of the iIlus'.ration. The cords may be made black, so that to the audience they are invisible. The faps are opened, exposing the apparently empty box to view. To fully convince the audience, a rod may be passed through the box, showing that there is no deception. The flaps are then closed and the lid brought down. The conjurer makes a few passes over the box, addresses his audience with the usual kind of patter, and presently opens the box once more, this time tilting it forward, so that the audience can look right into it from the open lid. And there, right enough, appears the mysterious lady in the box. For on closing the lid the drawer has moved forward into the box under the action of springs. In making his passes the conjurer has loosened two catches, with the result that, on opening the lid for the second time, only the central portion 0 of the cover is raised, leaving behind (unknown to the audience) a narrow marginal frame (shown at B in the detail drawing). It is this frame to which are attached the cords actuating the sliding drawer. Hence on lifting only the main portion of the lid, without its marginal fmme, the drawer is left within the box, and is exposed to view, with its contents, on tilting the box forward. There is, o'f course, now no difficulty in once more closing ti1e chest, fastening the catches, and displaying once more an empty box upon opening the lid. Variations and embellishments can of course be added to this performance to increase the spectacular effect. But it may 1e left to the ingenuity of the reader to devise these for himself. The apparatus her e described was designed by Mr. R. B. Smith, an Australian inventor now reside n t in New York, and the ac-e a mpanying drawing was prepared by the artist of the Scient i f i c A;UPHican from a small working mod 8 I kindly lent by the inventor. Mr. Smith is identified with numerous inventions chiefly relating to clocks and illusion apparatus for theatrical use.