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of the draw tube, notice that the tube is free from spider’s web or other obstruction, and that the stops are in their proper place. If all is correct, notice that a very slight rattle is just perceptible when the object-glass is gently shaken in its cell, and that a slight pressure with the points of the fingers will cause the object-glass to revolve. If this is not the case, slightly unscrew the counter-cell (a term given to the ring screwed in the cell against the object-glass); and turn the telescope towards a bright star, and focus with an eye-piece whose good quality is known. If the defect apparent in the image of a star was a flare on each side of the stellar point, the object-glass was too tightly held and when the counter-cell is slackened, the undue pressure will be removed and the flares will now not be seen. But if the flare is still on one side of the star, remove the eye-piece and place the eye in the focus of the object-glass, when the whole surface of the glass will appear illuminated, and any defect in its material at once rendered visible. Asa rule small scratches, or air-holes and bubbles, will only occasion a minute loss of light and are not likely to give rise to a flare. But should any of these defects appear very obvious on the illuminated glass and cannot be removed by wiping, affix pieces of sticking-plaster over them, one by one, until that which gave rise to the flare is covered. If these small coverings do not benefit, the glass is of uneven refraction, and stops of various shapes should be tried in front of the object-glass. Cut these stops of various circular, triangular, and oval openings and coverings, out of cardboard. Do not cover any more of the object-glass than is absolutely necessary to secure perfect vision, in order to preserve as much light as possible. If the edge of the object-glass bends back from having been worked too nearly its completed size, or worked after edging, a circular stop slightly smaller than the glass will render vision much more distinct. Revolve the object-glass by unscrewing the cell, and notice if the flare revolves too; if it does, and no stop can remove it, nothing can be done but to send the glass to a practical optician for his opinion. But if the flare or flares remain in one place and there is one spot in the diffraction rings brighter than the remainder, either the eye-tube does not point exactly to the centre of the object-glass, or the glass itself is not at right angles to the body-tube. The former defect can be identified by cutting out of card one circle the size of -the object-glass, and two circles to fit in the draw tube, all having central holes -J-inch in diameter. Place the large circle in front of the object-glass and the two smaller ones in the draw-tube. If the tube is in correct position, on looking through the one nearest the eye, the hole in the extreme circle will be seen exactly in the centre of the hole in the middle card. If it is ?iot, alter the direction of the eye-piece end of the telescope, loosening the screws which hold it, to allow of the same being done. But the flare will most likely be caused by the incorrect position of the object-glass, and can be corrected in the way before described, the cell most likely having received an accidental blow. In fact, several of the before-mentioned defects will never

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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 99,47 %.

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

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