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tube be shown (such as Figs. 17 and 18), it can be easily identified by moving the hand in front of the open end of the tube until it covers the dark crescent, and wherever this crescent is seen, it shows that that part of the mirror leans too far back, and therefore the screw behind it requires turning in, or the screw opposite to it unscrewed, in order to render the adjustment correct. In the Alt-azimuth cell it will be found necessary to slightly release the small clamping screws before shifting the large screws, but afterwards they should be firmly clamped, as on this greatly depends the stability of the adjustment. Take care that the adjustments continue perfect after the screws are all clamped.

A B. To rectify a Compound Error.

Should it happen that both speculum and flat require adjustment the latter had better be adjusted with the mirror removed from the tube. A sheet of white paper spread on the ground will serve to illuminate the end of the tube from which the speculum has been taken. The flat can then be adjusted to the illuminated speculum end, as in adjustment A3, and the mirror, when replaced, as in adjustment B. When the observer has learnt to distinguish between the non-adjustment of the mirror and flat, it is very easy to adjust the latter with the former in the tube, and even considerably out of adjustment. It is even possible to adjust at night on a lamp or other bright object.

To perfect the Adjustments.

When a star is viewed it should appear at focus, as a mere dot with one or two diffraction rings, its disc inside and outside the focal point should be perfectly similar, and when the expanded disc is in the centre of the field it should be seen, even when as small as possible, with the black dot perfectly central.

If, after the greatest care has been taken in adjustment, a flare should appear on looking at a star (say of the second magnitude) with an eye-piece of a high power, and the diffraction rings are not quite concentric, it can generally be rectified by turning the large mirror round in its cell a little at a time. If this does not remove the flare, the adjustment of the flat is not sufficiently correct, and must be altered by means of the screws at its back. If the flare is at the top or bottom of the star the flat must be very slightly revolved by the hand, after unloosing the counterpoise, and when correct, reclamping it. If at either side, namely, in the direction of the major axis of the flat and in a line with the tube, the flat must be altered by the long screw. It is always advisable to leave the telescope for a short time undisturbed, especially if, on first looking at a star, a flare should appear, as these appendages often vanish when the instrument has been for a short time in the air.

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