Abstract
LARGE TELESDOPES.—Muchhasheenwritten during the last few months with reference to the usefulness or non-usefulness of large telescopes. That the verdict is given in favour of the former is not at all surprising, for are we not far away from the limit, if there be one, beyond which larger instruments will be available? Dr. Common has many times pointed out the practicability of constructing large reflectors (his five-foot being a good example of the type of instrument he could enlarge), while the Lick instrument, the work of the Clarks, is really only a beginning of what will he done in large telescope building. For refractors it has many times been urged that the increase in size of lenses involves such a thickness that much light is thereby lost by absorption. M. Alvan G Clark, with reference to this particular point, says a few words in Astronomy and Astrophysics for April, in which he points out that such is not the case. Greater aperture means greater light-grasping power, and as it is quite unnecessary to considerably increase the thickness of the lens, the former predominates over the latter. With the forty-inch discs, he says, only a combined thickness of four inches is required, and with lenses of an object-glas of even six feet aperture a combined thickness of only six inches would be necessary. It is pleasing to hear through him that a steady improvement is being made in manufacture of glass, and that the present discs arc infinitely superior to the early ones, and “who knows,” as Mr. Clark says, “how soon still more transparent glass may be at hand.”
Article PDF
Rights and permissions
About this article
Cite this article
Our Astronomial Column. Nature 47, 616–617 (1893). https://doi.org/10.1038/047616a0
Issue Date:
DOI: https://doi.org/10.1038/047616a0