the same manner as if it had been reflected at the polarizing angle from a vertical plane parallel to x E.

Let us now consider what will take place by a variation in the length of the reflecting planes, the angular extent of the field of view remaining always the same. If A O E, A O E, Fig. 20, be two reflecting plates of the same breadth

A O, but of different lengths, it is manifest that the light which forms the direct sector must be incident nearer the perpendicular, or reflected at less obliquities in the short plate than in the long one, and, therefore, that a similarly situated point in the circular field of the shorter instrument, will have less intensity of light than a similarly situated point in a larger instrument. But in this case, the field of view in the short instrument is proportionally enlarged, so that the comparison between the two is incorrect. When the long and the short instrument have equal apparent apertures, which will be the case when the plates are A O E, A'O E', then similarly situated points of the two fields will have exactly the same intensity of light.

This will be better understood from Fig. 19, where o E may represent the long reflector and o'E the short one. Then, if these two have exactly the same aperture, or a circular field of the same angular magnitude, the rays of light which flow from two given points, p, n, of the long instru

ment, will be reflected at a certain angle from the points R, r; but as the points p', n', are the corresponding points in the field of the shorter instrument, the rays which issue from them will be reflected at the same angles from the points R, r, the eye being in both cases placed at the same point Hence it is obvious, that the quantity of reflected light will in both cases be the same, and, therefore, that there is no peculiar advantage to be derived, in so far as the light of the field is concerned, by increasing the length of the reflectors, unless we raise the eye above e, till every part of the pupil receives the reflected rays.

e.

There is, however, one advantage, and a very important one, to be derived from an increase of length in the mirrors, namely a diminution of the deviation from symmetry which arises from the small height of the eye above the plane of the mirrors, and of the small distance of the objects from the extremity of the mirrors. As the height of the eye must always be a certain quantity, E e, Fig. 17, above the angular point E, whatever be the length of the reflectors, it is obvious, that when the length of the reflectors is e o, the deviation from symmetry will be only P o', whereas when the length of the reflectors is reduced to é o, the height of the eye e E being still equal to e E, the aberration will be increased to P 0. This advantage is certainly of considerable consequence; but in practice the difficulty of constructing a perfect instrument, increases with the length of the reflectors. When the plates are long, it is more difficult to get the surface perfectly flat; the risk of a bending in the plates is also increased, which creates the additional difficulty of forming a good junction, on which the excellence of the instrument so much depends. By augmenting

the length of the reflectors, the quantity of dust which collects between them is also increased, and it is then very difficult to remove this dust, without taking the instrument to pieces. From these causes it is advisable to limit the greatest length of the reflectors to seven or eight inches.

CHAPTER VII.

ON THE CONSTRUCTION AND USE OF THE SIMPLE

KALEIDOSCOPE.

In order to construct the Kaleidoscope in its most simple form, we must procure two reflectors, about five, six, seven, or eight inches long. These reflectors may be either rectangular plates, or plates shaped like those represented in Fig. 1, having their broadest ends A O, B o, from one to two inches wide, and their narrowest ends a E, b E, half an inch wide. If the reflectors are of glass, the newest plate glass should be used, as a great deal of light is lost by employing old plate glass, with scratches or imperfections upon its surface. The plate glass may be either quicksilvered or not, or its posterior surface may be ground, or covered with black wax, or black varnish, or anything else that removes its reflective power. This, however, is by no means absolutely necessary, for if the eye is properly placed, the reflexions from the posterior surface will scarcely affect the distinctness of the picture, unless in very intense lights. If it should be thought necessary to extinguish, as completely as possible, all extraneous light that may be thrown into the tube from the posterior surface of the glass plates, that surface should be coated with a varnish of the same refractive and dispersive power as the glass.

If the plates of glass have been skilfully cut with the

diamond, so as to have their edges perfectly straight, and free from chips, two of the edges may be placed together, as in Fig. 17 (p. 49), or one edge of one plate may be placed against the surface of the other plate, as shown in the section of Mr. Bates's Kaleidoscope. But if the edges are rough and uneven, one of them may be made quite straight, and freed from all imperfections, by grinding it upon a flat surface, with very fine emery, or with the powder scraped from a hone. When the two plates are laid together, so as to form a perfect junction, they are then to be placed in a brass or any other tube, so as to form an angle of 45°, 36°, 30°, or any even aliquot part of a circle. In order to do this with perfect accuracy, direct the tube containing the reflectors to any line, such as m n, Fig. 2, placed very obliquely to one of the reflectors A o, and open or shut the plates till the figure of a star is formed, composed of 8, 10, or 12 sectors, or with 4, 5, or 6 points, corresponding to angles of 45°, 36°, and 30°. When all the points of the star are equally perfect, and none of the lines which form the salient and re-entering angles disunited, the reflectors must be fixed in that position by small arches of brass or wood A B, a b, Fig. 21, filed down till they exactly fit the space between the open ends of the plates. The plates must then be kept in this position by pieces or wedges of cork or wood, or any other substance pushed between them and the tube. The greatest care, however, must be taken that these wedges press lightly upon the reflectors, for a very slight force is capable of bending and altering the figure even of very thick plates of glass.

When the reflectors are thus placed in the tube, as in Fig. 21, their extremities a E, 6 E, next the eye, must reach to the