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Bubble Control. 

To obtain a Bubble. Holding the instrument as above described, rotate shade selector knob 4 to bring some figure other than nought to the opening 5. Rotate the bubble control knob 6 by moving the thumb downwards while looking through the eyepiece in a horizontal direction until a bubble appears in the field of view. This will probably be too large. Tilt the sextant slightly to the right and forwards so that the bubble appears about 10 o'clock in the field of view, and gradually ease back the control knob until the bubble has nearly disappeared; then tilt the sextant to the left to bring the bubble apparently about 2 or 3 o'clock, and finish releasing the tension, but do this slowly. There should now be a nice-sized bubble in the field of view. The most suitable size of bubble is between one-third and one-half the distance between the lines seen in the field of view. The control knob should he eased fully when it is not being used in the actual production of a bubble. To remove a Bubble. Slightly screw up the control knob; bring the bubble to appear about 10 o'clock in the field of view; ease back the control knob and the bubble should disappear. THE BUBBLE MUST BE REMOVED AND THE TENSION ON THE DIAPHRAGM RELAXED AFTER USING THE INSTRUMENT. When it is required to produce a bubble at night, the left Hand should be moved a little further up the handle and the back of the first joint of the forefinger of the left hand used to push the lighting switch 20 towards the body of the instrument. This enables the thumb to operate the control knob 6.