THE TELESCOPE CARRIER AND TABLE.
The arin carrying the telescope is firmly screwed to a large circle of wood E, which must be accurately turned or spoke shaved ; and, so that the clamp which bears on the right side may always have a good grip, the surfaces of the rotating table should be most carefully planed. This circular table has been added to save the continua] adjustment that is necessary with most amateurs' spectrometers, and if pains be taken to get it of even thickness, it will prevent a good deal of trouble. The block B takes advantage of the size of the table, and gives support to the tele scope as near as possible to the eyepiece. The collimator arm is fixed permanently to the large square base. It might have been placed lower than the telescope arm, but then the fixing of the prism table would have been a troublesome matter. Besides this, it will be seen that the centre of rotation of the telescope is adjacent to the scale, and therefore there will be less danger of eccentricity. The bolt N is also always held vertically. Fig. 779 shows the elevation, the lettering being the same throughout.
The middle tongue is turned so that it can be screwed on to the part of the collimator arm near L. The two side tongues turn under the table. Each is previously tapped to receive the screw s4, which by pressing against the under side of the table, makes it easy to get the prism accurately vertical. s, must be of very small pitch. Fig. 782 is a conventional view of the prism table. The collimator will now be described. The lens should be preferably corrected and consist of flint and crown glass cemented together. The focal length is 10 in. The tube may be of cardboard, but brass would be more satis factory. The lens may be fixed by first introducing a thin strip of cardboard, which is attached with glue or " Secco tine " to the inside to make a ring. Against this the lens can rest, whilst another outside ring will prevent it from falling out. The cardboard and the inside of the tubes should be first blacked with a mixture of lamp-black, size, and methy lated spirits.
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