Vlll INTRODUCTION. give access to the interior of the instrument and for vari¬ ous experiments. It can be drawn forward to suit a lens of longer focus. Its hood hides reflected and diffused light, and allows space for curtain and tinters. 11. The Sciopticon Curtain.—Turning the milled head at either side, gives the appearance upon the screen of a curtain rising, or falling, thus handsomely open¬ ing or closing an exhibition. It may also be temporarily closed at any time, to allow the attention to be directed to other exercises. The process of changing the pictures may be hidden from view by shutting off the light with the left hand ; then pushing the out-going picture into the left hand by sliding another into its place with the right ; and then flashing on the light with the right hand ; all of which may be sooner done than said. In any change of programme the awk¬ wardness of showing the “ full moon,” or the disk without a picture, may always be avoided by using the opaque cur¬ tain. This curtain also serves as a back cap for the objective, protecting the back lens from dust and light when not in use, as the front cap protects the front lens. 12. The Sciopticon Tinters.—The tinting-glasses are drawn up close behind the objective lens by means of rods terminating in knobs above. This lets the color down upon the screen, not with a sharp outline like the curtain, but with a gradual shading. With the blue tint partially drawn, this property gives to plain photographs of scenery a blue sky, shading off without abruptness down to the horizon. Slightly drawing up the blue, then the red, and then turning the button attached to the opaque curtain a little, fades away gradually the upper portion of the disk',