glass before the aperture =A O B=. The tube may be directed to the brightest part of the sky in the day-time, or in the evening to a candle, or an Argand Lamp, so as to transmit the light directly through the coloured fragments; but it will always be found to give richer and more brilliant effects if the tube is directed to the window-shutter, a little to one side of the light, or is held to one side of the candle—or, what is still better, between two candles or lamps placed as near each other as possible. In this way the picture created by the instrument is not composed of the harsh tints formed by transmitted light; but of the various reflected and softened colours which are thrown into the tube from the sides and angles of the glass fragments. When the pattern remains fixed in any position of the instrument, a variety of beautiful changes may be effected by making the end of the tube revolve round a candle or a bright gas flame, placed near the object-plate. The general pattern remains the same, but its colours vary both in their position and intensity, as the light falls upon different sides of the fragments of glass. In the preceding method of applying the objects to the reflectors, the fragments of coloured glass are introduced before the aperture, and pass across it in concentric circles; and as the fragments always descend by their own gravity, the changes in the picture, though infinite in number, constantly take place in a similar manner. This defect may be remedied, and a great degree of variety exhibited in the motion of the fragments, by making the object-plates rectangular instead of circular, and moving them through a groove cut in the cell at =M N=, in the same manner as is done with the pictures or sliders for the magic lantern and solar microscope. By this means the different fragments that present themselves to the aperture may be made to pass across it in every possible direction, and very interesting effects may be produced by a combination of the rotatory and rectilineal motions of the object-plate. When the object or objects are fixed, and the tube with the reflectors moved round a centre, as described in Chapter II.,[3] we have the same succession of symmetrical pictures; but in this case every alternate sector is stationary, and the same number in motion, the moving figures always changing their form, and assuming that of the figures in the stationary sectors, which of course change, while the ends of the mirror pass over the fixed objects. [3] See Chapter X.