Provide the telescope, if a refractor, with a dew-cap. Without this precaution dew is certain to gather upon the object-glass, with the result of stopping all observation until it is removed, and the accompanying risk of damage to the objective itself. Some instruments are provided by their makers with dew-caps, and all ought to be; but in the absence of this provision a cap may be easily contrived. A tube of tin three or four times as long as the diameter of the object-glass, made so as to slide fairly stiffly over the object end of the tube where the ordinary cap fits, and blackened inside to a dead black, will remove practically all risk. The blackening may be done with lamp-black mixed with spirit varnish. Some makers—Messrs. Cooke, of York, for instance—line both tube and dew-cap with black velvet. This ought to be ideal, and might be tried in the. . . Read More