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instrument be limited to celestial observation, in which an inverted image is of no consequence, the telescope will consist of a convex achromatic object-glass and some form of achromatic eye-piece whose power is equal to a single convex lens. In order to show an erect view for landscape observation additional lenses must be introduced between the object-glass and eye-piece, and these are generally combined with the eye-piece in one tube, and form what is called a terrestrial or day eye-piece. A day eye-piece should not be employed for celestial observation, as the extra lenses may occasion defective vision, and are certain to cause loss of light.

Very few adjustments are necessary with refracting telescopes, all that is necessary being to point the telescope to the object by means of the finder or otherwise, roughly focus by means of the sliding draw-tube, and finally sharply focus with the circular milled head which acts on the rack-tube. The object-glass and eye-pieces are truly centered and placed in correct adjustment by their maker, and nothing but violent usage can alter their position. The amateur should not attempt to alter or repair his eye-pieces, as only the very best brass turners ''and glass workers are employed on their manufacture, and should be entrusted with their repair. It may sometimes happen that when an observer has to carry his telescope from place to place, he will accidentally knock the cell of the object-glass and destroy its correct position at right angles to the tube. The effect of this will be that a star is not uniformly surrounded with diffraction rings of equal brightness, but there is a great concentration of light at one part. This defect can, with great care, be rectified as follows:—Slightly loosen the screws which hold the object-glass end to the tube, and with a piece of soft wood tap gently that part of the cell directly in front of the greatest brightness, or, in a contrary direction, that part of the cell exactly opposite ; continue to tap until the diffraction rings are perfectly even, and when this is so, screw up the screws. When thus adjusting, keep the object, which should be a bright star, always central in the field of view, and notice the even brightness of the rings both at and out of focus... Large object-glasses are provided with pull and push adjusting screws.

Should an object-glass, when first tried, appear defective in any way, carefully note every particular, as thereby the defect may be remedied. First see that the fault does not rest with the eye-pieces : endeavour to try them on other telescopes, and notice if the results are similar. Being sure that the lenses of the object-glass and eyepieces are quite free from dust and damp, revolve the draw tube holding the eye-piece, and if any defect revolves too, it will be evident that the eye-piece is imperfect, and it should be returned to the maker for correction. If the defect does not revolve with the eyepiece, but remain in one position, the fault most likely rests with the object-glass. First unscrew and remove the object-glass with its cell \ now, on looking through the telescope tube from the open end

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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 99,50 %.

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

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