It would be fallacious to suppose that the numerals 2, etc., on the dial represent accurately the percentage of oxygen yielded by the instrument when the indicator points to one of these figures, nor did Hewitt ever make such a claim. What the figures do represent is a number of holes in the wall of the mixing chamber, opposite to the aperture from the oxygen bag, which are uncovered one by one as the indicator moves over. The amount of oxygen which enters into the mixing chamber is regulated by the number of these holes uncovered, and also by the tension of the oxygen bag. If the figure on the dial is to be even a rough index of the actual percentage of oxygen present in the mixture, it is necessary to keep the tension reasonably constant, _i.e._ to regulate the flow of oxygen from the cylinder by manipulation of the foot key. In a brief administration for, say, a dental case, this is not necessary. It is sufficient to fill the oxygen bag once, and then turn off the supply. If, however, a long administration is required, a constant flow of oxygen of just the requisite amount must be secured. Administration. Put the lever at “air,” and fill up each bag to an equal and moderate degree of distention. Adapt the facepiece accurately to the patient’s face, and then push the lever to “N_{2}O”. After a few inhalations, move to 2 of oxygen; regulate the flow of nitrous oxide from the cylinder so that the N_{2}O bag remains slightly distended. Gradually move the indicator along the numerals until the figure 6 or 8 is reached at the end of about a minute or a minute and a half. Women and children require more oxygen than men. The former are easily cyanosed; if the latter are fed too generously with oxygen, they are apt to become excited. Take as your guide to the amount of oxygen required the colour of the patient, the type of respiration, and the size of the pupil. The _colour_ aimed at can only be learnt by experience, but is best described as a dull pink. _The Type of Respiration._--Too little oxygen leads to stertor and even stridor; too much oxygen, to a light almost noiseless respiration, which to the experienced ear is the certain precursor of a stage of excitement. Such a stage is clear evidence of too much oxygen having been given. _The pupil_ should not be dilated to anything like the degree seen with undiluted nitrous oxide. A moderate distention only is to be desired. _Full Anæsthesia_ should be reached in 100–120 seconds. It is marked by:-- (1) Dull pink complexion; (2) full respiratory movements with a stertor not exceeding that of gentle snoring; (3) eyeballs rotated downwards; (4) moderately dilated pupils; (5) loss of conjunctival reflex; (6) corneal reflex present but not very active.