After it has been properly dried the film again is wound upon a metal spool, put in an airtight box and sent to the assembling room, where the various scenes that go to make up the picture play, taken at different times and on different rolls of negative, are joined together in their proper order to make a complete play in a single roll about one thousand feet long. After the negative film is developed, dried and wound upon a metal spool it is sent to the printing room, where positive prints are made from the original impression. Right here it may be well to say that on a negative film or plate in any kind of photography white appears black and black appears white--hence the name negative. The paper or film upon which the print is to be made turns black wherever the light strikes it, so that when the negative is laid over the positive and exposed to a strong light the rays quickly penetrate the white spots on the negative and turn the corresponding spots on the positive black. The light does not penetrate the places on the negative which are black, and consequently leaves those places on the positive white. The result is that the positive shows the image just as it appears to the eye. The principle of printing positive films, then, is the same as the principle of making photographic prints or positives from ordinary still photography plates or films, but of course it is far more complicated because of the mechanical difficulties of bringing the two long, unwieldy strips of film together in the proper position. The whole process is carried out by a machine which takes the place of the printing frame into which the amateur so easily puts the still-life photographic plate and printing paper. There are several motion-picture printing machines in use in this country, but in their central idea they are similar, as they all pass the negative and positive films before a very bright light so that the impressions on the negative are transferred to the positive. The invention of this machine was a necessity for the commercial success of motion pictures, for obviously it was impossible to lay a strip of film several hundred feet long and about an inch wide in a printing frame over a positive film of the same length and width. [Illustration: A MOTION-PICTURE PRINTING MACHINE A-A´--Rollers for negative film. B-B´--Rollers for positive film. C--Film gate where positive is held over negative for printing. D-D´--Negative film. E--Unexposed positive film. E´--Exposed, or printed positive film. F--Light which, shining through film gate, imprints image of negative on positive.] The explanation of one printing machine will suffice to indicate the general principle. Some of the machines are worked by hand power, but in the larger reproduction studios electric power is used practically altogether for running the battery of printing machines.