KALEIDOSCOPE, a name compounded of two Greek words (saran and crtaiwor), and denoting the exhibition of beautiful forms, is the designation of an optical instrument which was invented by Dr. (now Sir David) Brewster, and patented by him in 1817.
About three years before that time Sir David Brewster, being engaged in making experiments on the polarisation of light by reflec tion, from plates of glass, observed that when two plates were inclined to one another, and the eye of the spectator was nearly in the pro duced line of the common section of their planes, the farther ex tremities of the plates were multiplied by successive reflections so as to exhibit the appearance of a circle divided into sectors, also that the several images of a candle near those extremities were circularly disposed about the centre ; and these circumstances sug gested to him the construction of an instrument of the kind above named.
It may be observed, however, that the multiplication of the image of an object by successive reflections from mirrors inclined to one another had long before been a subject of investigation in treatises on optics ; and both Baptista Porta and Kircher had given descriptions of instruments consisting of mirrors united at two of their edges, which, being opened like two leaves of a book, were capable of multiplying the images of objects. Bradley also, about the year 1717, constructed an instrument consisting of two plates of glass inclined to one another, which being placed on a drawing, with the line of section perpendicular to the paper, exhibited to the eye several images of the figures, dis posed by successive reflections about a centre. But the optical inves tigations alluded to are very remotely connected with the properties of the kaleidoscope ; and the application of the latter to objects which may be moveable and situated at any distances from the observer, render Brewster's instrument very different from and far superior to the simple contrivances of Porta, Kircher, and Bradley.
The essential parts of the instrument consist of two plane mirrors of glass, having their posterior surfaces blackened In order to prevent any reflection of light from thence ; mirrors of polished metal would, however, be preferable : each mirror is from six to ten inches long, and of a trapezoidal form ; the larger end about an inch and a-half long, and the shorter end about three-quarters of an inch ; and the two arc placed in contact with one another at a long end of each, so as to form a dihedral angle, the like ends being placed together : the object to be viewed is disposed contiguously to the larger ends, and the eye should be near the opposite extremity, but a little above the line of contact. The effects produced by the reflections of the light
may be understood from the following considerations : Let A 0 B c, in the first of the figures, be the two extremities of the mirrors on the side farthest from the eye of the observer, which is supposed to be near the opposite extremity of the line of section passing through o perpendicularly to the plane of the paper. These lines A c, n o, and the sectors! space between them (which in the figure Is one-eighth part of a circle), will be visible by rays coming directly to the eye; and, at the same time, rays from the line A C falling on the mirror B C at a certain angle of incidence will, on being reflected from thence to the eye, give rise to the image c a of that line ; in like manner rays from the line B e falling on the mirror A 0 at an equal angle of incidence will, after reflection, give rise to the image a b of the line. These, with the intermediate rays, produce the first re9ected sectors is c a and A c b. Other rays from•the sector A a b at the surface of the mirror A C will fall on the mirror B c; and, while a portion of them arrive at such angles of incidence as to be reflected to the eye and, produce the perception of the sector a c b', another portion of them will be reflected back to the mirror A e at such angles of incidence as to be re-reflected to the eye and cause the perception of the sector a'c b". In a similar manner the rays first reflected from B will, by subsequent reflections, give rise to the perceptions of the sectors b c a', e.