Mirror

Optical Illusions.

Many optical illusions can be produced by the aid of plane mirrors, usually unsilvered or semisilvered or platinised ones are required. One of the commonest devices is to vary the illumination of the objects, of which some are placed behind and some in front of the mirror. An eye will see only one of these, if it is illuminated strongly and the other left dark.

Optical Illusions with a Concave Mirror.—A concave mirror AB' A'B (fig. 4)., will produce at QQ' a real image of an object at PP'.

If R and L are the right and left eyes of the observer, the right eye will see the image by rays coming from the portion BB' of the mirror, and the left eye by those coming from the portion AA'. If therefore the mirror is large enough to extend from A to B, the image will be seen by both eyes as though the light were actually coming from an object at QQ', and seems to be perfectly real and tangible. Thus an apple supported in a box UVWX (fig. 5) against a black velvet back ground, and lighted by a lamp L, can be projected by a concave shaving mirror among other fruit R on a plate CD. It will disap pear if the observer moves to one side, or L is turned out.

Very striking effects are pro duced if water or mercury is allowed to run out from a tube, and splash into a glass dish ; then if a second actual tube and glass dish are put to coincide with the images of the tube and dish, it will appear as if the liquid is falling upwards.

The Tetrahedral Prism.

Let YOZ, ZOX, XOY, be three plane mirrors, mutually perpendicular, intersecting in OX, OY, OZ. Describe a sphere with centre 0, and cutting the mirrors in great circles YZ, ZX, XY (fig. 6). Let a beam of light parallel to PO fall on the inside faces of these mirrors. Each of the mirrors will intercept and reflect part of the beam, which will then fall in turn on each of the other mirrors. Draw a great circle through XP, cutting YZ in M and measure MP' equal to MP, the light reflected in the mirror YOZ will travel parallel to P'0 after reflection. As the angle at M is a right angle, it is obvious that OP' will make the same angles with OY and OZ that OP does, and that the angle it makes with OX is the supplement of that made by OP with OX. So reflection with any mirror changes the angle with the normal to that mirror into its supplement ; successive reflection in all three mirrors will change all the angles made by OP with the normals OX, OY, OZ, into their supplements, i.e., the beam will return exactly along its path.

As the final direction of the beam is simply the opposite of the original direc tion, it is not affected by rocking the system of mirrors. The mirrors are usually in the form of a tetrahedral prism, i.e.,

corner of a cube cut off by a plane meeting the edges at equal dis tances from the corner. On looking into the prism each eye sees itself in a line with the vertex, and no change seems to take place when either eye is closed.

Astronomical Mirrors.

The earlier astronomical mirrors were made of speculum metal, but they were exceedingly difficult to cast owing to strains produced in cooling, and the surface was liable to be imperfect. For these reasons glass has been almost exclusively used for the large mirror since the discovery by Liebig of the method of front silvering it. Very large mirrors, ground from single slabs of glass, have been made successfully. Apparently the limit of size has been nearly if not quite reached, for it is doubtful if the annealing of much larger and thicker slabs would be possible. Professor Ritchey who was responsible for the construction and figuring of some of the largest mirrors, has recently, however, described a built-up cellular glass structure, by which he believes that mirrors up to as large as 3o feet in diameter could be made. An upper and a lower plate of glass of moderate thickness are connected together by vertical ribs of about the same thickness. This forms a structure of comparatively light weight and great rigidity, which will retain its form at all temperatures. He believes it should be sufficiently rigid to be ground and figured to the required degree of accuracy to form the optical image.

The first rough working of these mirrors may be done in one of several ways. They may be turned with a diamond; or they may be ground with a grindstone made of carborundum or alundum, both glass and stone being rotated; or an iron ring fed with grinding material may be used. This grinding tool is a disc of iron of about the same size as the mirror itself, and it is formed with a convex surface of the same radius of curvature as the mirror is to have. In any case the final grinding and smooth ing are done with this tool, the mirror resting upon it, face down. After the whole surface has been brought to the desired curve, the rough grinding is complete. It next has to be "smoothed." The smoothing is produced by continuing the grinding with a series of finer and finer grinding materials. After the smoothing is finished the surface should be grey when dry, and appear perfectly free from any pits or scratches, even when it is examined with a magni fying lens. The next stage is the polishing, which is done with moist rouge (ferric oxide), washed to remove any coarse particles. For this part of the process the tool is covered with a thin layer of semi-elastic material, pitch mixed with ashes or wool.