MOVING PICTURE APPARATUS 403 Ch. XII that the arc can be watched. This window should be of fairly targe size and directly opposite the crater of the arc. The glass should be dark enough so that the eyes will not be tired by the too I groat brightness and yet light enough so that the whole of the hot carbon ends can be seen. Another convenient way to observe the arc is to bore a fine hole ln the side of the lamp-house away from the operator. This acts hhe a pinhole camera and an image of the arc is seen on the opposite ^l- A sharper image of the arc can be formed by using a long f°cus lens over an opening in the wall of the lamp-house to focus an ^gc of the arc upon the wall. A spectacle lens of about 25 cm. ^l0 in.) focus (4 diopters) will answer. The lens may be held by an.v convenient clamp but must be adjusted for distance to get the sharpest image, otherwise it is no improvement over the simple Pinhole. The lamp-house should be well ventilated as from ^ to 2 kilo¬ watts of jxïwcr. .7 to 3 horsepower, is converted into heat. While arc is going there must be some way for this heat to escape, °thonvisc everything inside would melt. One of the principal Causcs of condenser breakage is poor ventilation of the lamp-house. Tho best ventilation is secured by having holes permitting air ^culation but no escape of light, at the top and near the bottom the lamp-house. The back of the lamp-house is sometimes rei*ioved. in many places the fire underwriters or the city, require that ^t-se ventilating holes be covered with fine ware gauze, to prevent ^^trks flying out. This requirement was invented by someone had the mistaken idea that an arc lamp was a fiery volcano, c>i'niting out sparks and lava in all directions instead of a quiet, ^“11 behaved sort of thing. It is true that a minute spark some- x'*">es does fly up, but it is so light that it cannot do any damage. ‘■'*'1 y small piece of the carbon tip which breaks off will fall to the ^ttom of the lamp-house where a suitable tray should be pro- x*3edtocatchit. This tray is also useful to hold the short pieces of carbon just taken out of the lamp when new carbons are put in. § 568. Condenser.—The condenser is usually in a box which is ^stened to the lamp-house and moves with it. In front of the con-