The eyepieces are of triplex glass in metal rings with rubber gaskets. In pressing the rings home, the rubberized stockinet is turned and held securely so that there is no possibility of pulling them out. The angle tube containing the outlet valve and the connection to the corrugated tube connecting with the canister is the same as with the latest model R. F. K. mask. The only difference as regards the corrugated tube is that a greater length is needed with the new carrier under the left shoulder. The total length of the tube for this model is about 24 inches. On the inside of the facepiece and connected to the angle tube inlet is a butterfly baffle of rubber, so arranged that the incoming air is thrown upward and over the eyepieces, thus keeping them clear no matter how much the exertion or what the temperature, except in certain rare cases when the temperature is down at zero F. or below. [Illustration: FIG. 62.—1919 Model American Mask.] CANISTER The canister is radically different from the canisters used in the R. F. K. and earlier types. In the first place, it is longer, the total length finished being 8 inches. It has two inlet valves at the top end protected by a tin cover instead of the single inlet valve at the bottom of the earlier types. The two inlet valves are each ⅝ inch in diameter and are made up of square flat valves on the end of a short rubber tube. The rubber tube is fitted over a short metal tube. Gas-tightness is obtained both by the pressing of the valve against the round edge of the metal tube and by the pressure of the edges against each other. These valves, while delicate, are proving very satisfactory, and being simply check valves to prevent the air going back through the canister, they are not vital. In case of failure, the eyepieces would fog somewhat and the dead air space be increased by that held in the inlet tube. The canister consists really of two parts—an outer casing that is solid and an inner perforated tin casing. Around the perforated tin is fitted a filter of wool felt ³/₁₆ of an inch in thickness. This wool felt is very securely fastened by turning operations to solid pieces of tin, top and bottom, so that no air can get into the chemicals without passing through the filter. Thus the air coming through the inlet valves at the top circulates around the loosely fitting outside corrugated case to all parts of the filter and after passing through the filter continues through the perforations of the tin into the charcoal and soda-lime granules.