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flat is perfectly central, the nuts should be screwed up tightly, and this adjustment will be complete. Should the wide part be equidistant from two of the nuts, screw both these, releasing the third. Care should be taken that the springs obstruct as little light as possible; they should, therefore, oppose their edges to the mirror end of the tube, and not to the sides.

A2. To centre the “Flat” to the Rack and Draw Tubes.

In order to correctly centre the eye to these tubes, resource should be had to the adjusting-piece supplied with each instrument: this consists of a brass disc with a very small central hole, and is placed in the same position as an eye-piece. The flat mount and counterpoise having been removed from their ordinary position, set the face of the flat as nearly as possible at an angle of 45 °, which may be effected thus. Draw on cardboard a square figure with sides of 3 or 4 inches, join either of the corners, and the diagonal line will be exactly on an angle of 450 to all the sides. Use this diagonal to set the face of the flat at an angle of 450 to a line drawn as a continuation of the axial pin of the flat mount. When the angle of the flat is correct, replace it in the tube, and, looking through the adjusting-piece, revolve the flat till as much as possible of the open speculum end of the tube is seen reflected in it. Should, however, it be found impossible to show the complete end of the tube, slightly loosen the brass screw nuts and shift the springs carrying the flat mount, backwards or forwards, till the complete end is seen reflected in the centre of the flat, and the flat itself is exactly concentric to the circular edge of the draw tube. When this is so, screw up the nuts as equally as possible, so as not to disturb the centering of the flat which formed the last adjustment.

C1. To set the Rack and Draw Tube in position.

Cut out a piece of white card or thick paper 3 or 4 inches in diameter, and punch a half-inch hole in the centre of it. Having removed the adjusting piece, hold the card in front of the eye, looking through the small hole. Now let the eye and card be removed from the draw-tube until its circular opening illuminated by the white card is seen reflected in the centre of the flat, the margin appearing apparently dull. If the flat be at 450, and the rack-tube at right angles to the body, the reflected white circle will remain in precisely the same position when the draw-tube is pushed as far in as possible as at its greatest extension. Should the white circle alter its position on moving the rack and draw-tubes, they are not at right angles to the body-tube, and the direction the circle moved should be noticed, and the rack-tubes altered accordingly by fixing thin washers between their plate and the tube: often slightly loosening

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Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 99,36 %.

La langue de reconnaissance de l'OCR est l'Anglais.

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