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one of the attaching screws will even suffice. During this adjustment the speculum must be in the tube and also in ordinary adjustment, which may be effected as described on page 28, or adjustment Cl can be performed last of all.

Ordinary Adjustments.

The correctness of the primary adjustments having been verified, the observer should particularly notice the compound reflections of the speculum and flat seen through the adjusting-piece, as by a thorough knowledge of these he will be enabled at once to identify any irregularity, and then quickly correct it. The flat having been placed in the centre of the tube will prevent an equal area of parallel rays from falling on the mirror, therefore there is a central spot on the mirror, from which no rays are reflected, and this spot, which will of course appear dark when the mirror is illuminated, is of the greatest assistance in the ordinary adjustments.

To place tlie Mirror into its Cell.

The back of the mirror and the face of the metal ring on which it rests having both been carefully worked flat, and when practicable ground together, many mirrors will be found to perform admirably resting on the metal alone; should, however, a thick piece of felt or flannel be interposed, definition may sometimes be greatly improved. If there should be any mark on the mirror to indicate that it should rest on a certain part of its edge, be careful to place the mirror as indicated. Should it be necessary to elevate the speculum end of the tube, in order to view terrestrial objects, the speculum should be prevented from falling forward out of its cell by a brass ring or some clips; care should be taken that these do not quite touch the surface of the mirror, but are about the thickness of an ordinary sheet of writing paper from it. The mirror being in its cell, and perfectly free from strain on any part of its surface, it may be placed in the tube.

To place the Speculum in the Body Tube.

There are two kinds of speculum cell fittings usually employed, one of which is only applicable to Alt-azimuth instruments, but the other to any construction, and therefore, being more important, is described first. Attached to the end of the body tube are three screw-pins, each of which passes through a corresponding hole in the frame which supports the speculum cell. Screwing on each pin are a couple of brass nuts—one on each side of the frame—by means of which the frame can be delicately moved or firmly kept in any position. By detaching the outer nuts the speculum and cell can be easily removed arid replaced without altering their adjustment.

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