Once the eye of the anæsthetist is trained to its use, this is a very simple means of gauging the relative proportions of oxygen and nitrous oxide which are being delivered, and manipulation of the cylinder heads combined with visual inspection of the sight feed enable one to strike the right proportions very easily. The nitrous oxide is usually kept at a constant pressure sufficient to ensure bubbles, not only from all the side holes, but also a few from the open end of the tube. The oxygen pressure is begun at the point where there is a little bubbling from the top hole only, and is gradually increased until there is a full supply from two holes, occasionally a little even from a third. Messrs Coxeter have recently brought out two sight feed machines designed by Mr Leonard Boyle and Dr Geoffrey Marshall, of which the latter is shown in Fig. 22. This apparatus may be put up in either portable form or a larger type for use in hospitals. An ether chamber is provided for use when necessary in either type. As originally introduced, the remainder of the apparatus consisted simply of an ordinary two gallon bag, Barth 3-way tap, and rubber facepiece. With such an appliance, it is not possible to secure “positive pressure, a point which the author brought to the notice of the makers. Messrs Coxeter are willing to supply a facepiece and expiratory valve which obviate this defect, being supplied with a mica expiratory valve the lift of which can be controlled. There should be no inspiratory valve. Administration of Gas-oxygen for the purposes of Major Surgery. The patient is prepared with the same scrupulous care as if ether or chloroform is to be administered. Half an hour before operation, morphia gr. ⅙ and atropine gr. ¹⁄₁₀₀ are given hypodermically. The anæsthetist before beginning administration, must look over the apparatus most carefully and satisfy himself that every part of it is in perfect order, and that a sufficient supply of both gases is at hand. The inhalation is begun by the use of nitrous oxide alone, given “on the valves,” and at no great pressure. After a few breaths, oxygen is added very guardedly, the proportion being steadily raised during the first two minutes: after that point, a further increase will not be necessary until several more minutes have elapsed. The pressure at which the mixture is being given is also steadily increased and should reach the maximum permissible within a few minutes. A useful plan is to allow the flow of gases to remain constant, but to close the expiratory valve at frequent intervals for about forty to sixty seconds at a time. During this period of complete re-breathing the tension in the supplying bag will of course rise, falling again slightly when the expiratory valve is allowed once more to come into action. As soon as the tension falls appreciably, the valve is again closed down.