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be found in the work of a careful optician, but will result from an accidental circumstance.

The lenses of a telescope should never be cleaned, unless it is absolutely necessary. If dust has settled, first brush it off with a clean dry camel’s hair pencil, and then very gently wipe clean with a piece of very fine wash-leather, or a soft silk or cambric handkerchief. Any material thus employed should be specially kept for this purpose alone, and when not in use carefully protected from dust. Should the lenses require to be removed from their mounts for cleaning, or otherwise, take care that they are correctly replaced. The edges of an object-glass generally have marks which should be placed together. It will hardly require notice that the convex glass always is next the object viewed. Before screwing in any cell or mount, give half a turn in a contrary direction, when the first thread will generally drop home, thus ensuring truth, and avoiding the risk of the threads of the screw being spoiled.

As it is impossible, with our present knowledge, to perfectly correct the secondary spectrum of an object-glass, a slightly coloured image of a bright object is unavoidable. A good glass should give a very slight tinge of bluish purple at focus, and rather more purple within and green without the focal point. Should, however, the object have a red or an orange colour at the focus, and within the focus the expanded disc show a purple centre and red margin, and without a red centre and purple margin, the marginal colours being the most apparent, the object glass is under-corrected for colour. A slight separation of the lenses may tend to diminish this. In an overcorrected lens the colours will be exactly in the reverse order, but nothing can be done by the amateur to correct them.

I have now mentioned all the principal defects of a telescope that an amateur may attempt to remedy, but it will be far better policy for him to ask a practical optician to attempt the correction than to try it himself, as he may do irreparable damage. None of the remedies here mentioned can do the slightest damage if carefully applied.

An optician can generally remove any error except irregular density, by repolishing one or more of the glasses, but as they have been edged from worked discs considerably larger than the object-glass itself, there is a risk of injuring the centering and figure of the lenses if they are worked after being edged. It will therefore be better for the possessor of an inferior object-glass to procure a new one of similar size and focus, or if the imperfect glass is of large size, and it is necessary to have it re-worked, let it also be re-edged to a smaller cell. ~ '

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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 99,63 %.

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

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