If a carriage about two feet long is used, the carriage may be moved alternately from right to left, and while one side contains the cards which are being reflected on the screen, the other side may be loaded with the cards next desired. On account of its length the carriage will hold curve cards for several succeeding years. By pushing the carriage slowly across the lantern the fluctuation in any curve may be shown for as long a series of years as may be desired. Simple spring clips on the carriage may be used to hold the overlapping cards in their

304 GRAPHIC METHODS

exact position. Not only are the curves themselves shown on the screen, but the whole face of the card is seen so that the figures for any points on the curve Avhich are of special interest may be read directly from the screen.

If the information or record room is large enough to serve for holding conferences the lantern and the screen may be installed permanently as part of its equipment. Ordinarily, however, it will be found best to have the record room more private than any room used for general conferences can be made, and the lantern would be part of the equipment of the conference room rather than of the record room. With this arrangement it will be necessary to take the curve cards from the record room to the place where the lantern is installed. The file for the curve cards (see Fig. 217) had better be ecjuipped with spring locks so that there will be no danger, when the file is carried, of drawers slipping out and spilling the cards. A rod arrangement is never desirable with such cards as these, for the rod would spoil the bottom portion of the card and would also make it impossible to lift cards out for quick reference or comparison.