It will be observed that all of the above tests are concerned only with the efficiency of the absorbent and its packing in the canister. No attempt was made to determine the comfort and general “feel” of the mask. For this purpose field tests were devised, covering periods from two to five hours. The first test was a five-hour continuous wearing test. It was assumed that any mask which could be worn for five hours without developing any marked features of discomfort could, if the occasion demanded it, be worn for a much longer period of time. A typical test follows: 8:00 to 8:30 Instruction and adjustment of gas mask. Gas-chamber tests 8:30 to 9:30 Games involving mental and physical activity 9:30 to 11:30 Cross-country hike with suitable periods of rest 11:30 to 12:00 Tests of vision 12:00 to 12:30 Games to test mental condition of subjects 12:30 to 1:00 Gas-chamber fit test [Illustration: FIG. 79.—Hemispherical Vision Chart.] Vision was tested by means of a hemispherical chart (Fig. 79). This chart was 6 ft. in diameter and was constructed of heavy paper laid over a wire frame. A hinged head rest was provided for holding the subject’s head firmly in position with the center directly between the eyes. The subject wearing the mask took up his position, and with one eye closed at a time, indicated how far along the meridian of longitude he could see with the other eye. The observer sketched in the limit of vision by outlining the perimeter of the roughly circular field allowed by each eyepiece. The intersection of the two fields gave the extent of binocular vision possible with the mask. Various other tests were also used, in order that the extent and nature of the vision could be accurately determined. Aside from the problems of comfort, protection, vision and other important features of gas mask efficiency, the question arose as to whether certain designs of masks or canisters were mechanically able to withstand the rough treatment they were certain to receive in actual field service. A test was, therefore, developed to simulate such service as transportation of masks from base depots to the front, carrying of supplies and munitions by men wearing masks in the “alert” position, exposure to rain and mud, hasty adjustment of masks during gas alarms and typical mistreatment of masks by the soldiers. All these tests were of great value in the development of a good gas mask. CHAPTER XV OTHER DEFENSIVE MEASURES PROTECTIVE CLOTHING