The other devices for holding materials in position consist of two devices for holding motion picture films in position so that they may be studied with the least amount of effort possible. [Illustration: FIG. 17 These chairs are of type four, devised to relieve fatigue caused by vibration. Besides the chairs, foot-rests were devised to hold the feet without any vibration from the floor; and, also, special treadles.] [Illustration: FIG. 18 This chair is of type six, which modifies a device already in use, so that it will become a more efficient device for eliminating fatigue. The chair shown was, ordinarily, fairly comfortable, but the legs had become worn with time. It has, as shown, been raised to that height which is most comfortable for the worker. The work-bench, in this case, could not be raised so that the work could be done either standing or sitting. The problem was to have the sitting work done with the least unnecessary fatigue possible.] The transportation devices illustrate the principle of gravity, and also the principle of constant and careful adjustment of the transportation to the worker. How to Use the Devices. The devices of the fatigue museum are useful rather as suggesting devices than as object lessons. If your problem is to enable seated work to be done standing, raise your work-bench to the standing level, and put your work chair on stilts with casters, provided the work is not of a kind that requires a chair against which one can push. If your problem is to enable work that has been done standing to be done sitting, construct a chair that will bring the worker to the desired height. If your problem is to reduce vibration, put springs under the four legs of your chair. If your problem is simply to make sitting work more comfortable, be sure that the chair is of the proper height; that the seat slopes right and has a rounded front edge; and that, if it has a back, it is one that does not interfere with work. If the chair is too high, saw off the legs; if too low, add wooden blocks. Chairs of this type, as actually used by the workers, will usually offer suggestions as to what needs to be done.