AIDS TO VISION.
[To THE EDITOR OF THE " SPECTATOR."] SIB,—As a reader of the Spectator for many years, may I be allowed to make a correction ? In your last issue the writer of a highly interesting article on "Aids to Vision" says : "Probably no further attempt will be made to construct an instrument as large as Lord Rosse's six-foot reflector." As an amateur astronomer with a humble six-inch reflector, I write to call your attention to an interesting announcement in that excellent periodical, Knowledge, for December, p. 613 (also in Nature-a fortnight ago),—viz., that through the munificence of Mr. .1. D. Hooker, of Los Angeles, Professor Hale is arranging for the construction of a glass mirror 100 in. in diameter (8 ft. 4in., or 2-54 metres), focal length about 50 ft. The grinding of this parabolic mirror will take the best part of four years. Refrigerating machinery will be necessary in Order to obviate the effects of changes of temperature. Pro- fessor Ritchie is making a mirror of 60 in., and astronomers will look forward with much interest to the completion of the two telescopes.—I am, Sir, &c., 94 Fellows Read, Hampstead, H. W.
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