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mcintosh battery and optical co., Chicago, ill., u. s. a. 83 These battens should be fastened on with screws, which must not pro- J ct through the board, otherwise they will wear holes in the bag. To prevent the bag from slipping out from under the pressure board when he weight is adjusted, two leather straps may be fastened upon the pper edge of the board, and be brought down across the bellows end of the bag, and fastened to the floor. The latter must be free frdm sand, nails or splinters, as these are liable to injure the bag A sharp tack, carelessly left under it, has been known to puncture it under great pressure. puncture it, THE SCREEN. The white surface that receives the projected picture is called the screen It may be a white finished wall, or a white cloth properly °T ed ' I™ 6 b r aCk ° f a . wa11 ma P> lf clean and white, may be used, hatever form of screen is selected must be perfectly smooth, and Us surface must be parallel with the front of the objective. It may be of any size, but for convenience should be not more than a foot or two larger in diameter than the largest size disk to be employed. Exhibitors who wish to use both large and small halls find it conve- ar^ n V?p Ca K ry * tW ° , scree " s of diffe rent sizes. Those we manufacture are of the best quality of heavy bleached cotton; they have a firm border of thick, non-elastic webbing, stitched entirely around the edge, with brass rings about one inch in diameter at intervals of two j? crew eyes of sufficient size to hold the guy ropes are fastened n the floor and ceiling, and the screen is drawn smooth and free from wrinkles. Very careful adjustment is required when the microscope attachment is employed. The four corners of the screen must be equi-distant from the objective. A convenient method of ascertaining a correct position is to tie a long cord to the objective, and measure the distance to each corner of the screen. th r f the r 61 ? tei i of the screen is on a higher level than the lantern, the front of the latter must be elevated until the disk of light will fall on the center of the screen. Sometimes it is necessary to tilt the upper part of the screen forward to make its center parallel with the Lantern Lens. All of our screens are provided with two very strong rings, capable of sustaining enormous strain, at each corner. A few directions as to the easy way of hanging a large screen will not come amiss to the amateur; 3 ropes are necessary for hanging a screen in this manner: purchase 200 feet of No. 7 or 8 sash Weight cord, which, if an A No. 1 quality, is capable of standing a tensile strain of 600 pounds. One rope you never cut; the other one you cut in half; this gives you three pieces, one 100 feet long and two 50 feet long. Lay your screen on the floor under the place where you wish to hang it. At one side of the hall place a strong screw eye in the floor or base boards, of one- fourth inch wire at least; to this ring fasten one end of the 100