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ADJUSTMENTS OF A NEWTONIAN TELESCOPE.

* The necessary conditions of correct adjustment are as follows :—

A. The flat mirror must be so placed that it receives all the rays of light reflected from the speculum, and so adjusted that it bends them at an angle of 45° to the eye-piece.

B. The speculum must be so adjusted that it reflects all the light it receives to the flat mirror.

C. The eye-piece must be so placed exactly opposite the flat mirror that it receives all the light reflected from it.

The correctness of these three adjustments is of the greatest importance; in fact, a telescope cannot perform satisfactorily unless they are perfect. The reader will notice that the terms placed and adjusted are both employed, the former comprehends what may be called primary, and the latter ordinary adjustments.

Primary Adjustments.

It is the duty of the optician to see that these adjustments are perfectly correct in a telescope before it passes into the possession of the amateur, who however should ascertain that they are so, and be able to correct any defect in them that may be discovered arising from accident or otherwise.

The primary adjustments may be tabulated thus :—•

A1. To centre the flat in the body-tube.

A2. To place the flat at an angle of 450 exactly opposite the eye-piece. *

C1. To set the rack and draw tubes at right angles to the body-tube, so that the eye-piece they hold is exactly opposite the flat.

A1. To centre the Flat in the Body-Tube.

Cut out in cardboard two .discs that will fit tightly in the body-tube, one with a central hole of about ^ of an inch, the other with a hole of about an inch larger in diameter than the flat; place the latter at the open end of the tube, and the disc with the small hole to occupy the space from which the speculum in its cell has been removed. Now on looking through this small hole towards the other, if the flat is central it will stop out the centre of the larger hole, and have a ring of light equally all round it; should this not be the case, but the bright ring be wider at one side than at the other, notice which of the three screw nuts (which stretch the springs supporting the flat mount) is nearest the wider part of the ring, and screw it up slightly, of course releasing the others as you proceed. When the

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