Our Astronomical Column

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    Abstract

    A NEW ACHROMATIC OBJECT-GLASS.—It is well known that in consequence of the irrationality of dispersion the nominally achromatic object-glass is really very far from achromatic. There is always a residual colour, frequently called the secondary spectrum, so that the images of bright stars are surrounded by halos of blue and red light. For this reason a refracting telescope designed for visual observations cannot be employed for photography. Many attempts have been made to correct this colour aberration of the achromatic lens, but the plans hitherto suggested have never been practically adopted, owing to difficulties of construction, or to the imperfect durability of the glasses employed. Mr. H. D. Taylor, optical manager to Messrs. T. Cooke and Sons, has recently taken up the question, and he appears to have come very near to a practical solution of the problem. He has aimed at producing an objective which shall be (1) almost perfectly achromatic; (2) equally well corrected for photographic purposes as for visual purposes; (3) capable of practical construction in large sizes; and (4) of ordinary durability.

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