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 case of the dew-cap by the observer. Finders are rarely fitted with dew-caps, but certainly should be; the addition will often save trouble and inconvenience. Be careful to cover up the objective or mirror with its proper cap before removing it into the house. If this is not done, dewing at once results, the very proper punishment for carelessness. This may seem a caution so elementary as scarcely to be worth giving; but it is easier to read and remember a hint than to have to learn by experience, which in the case of a reflector will almost certainly mean a deteriorated mirror film. Should the mirror, if you are using a reflector, become dewed in spite of all precautions, do not attempt to touch the film while it is moist, or you will have the pleasure of seeing it scale off under your touch. Bring it into a room of moderate temperature, or stand it in a through draught of dry air until the moisture evaporates; and should any stain be left, make sure that the mirror is absolutely dry before attempting to polish it off. With regard to this matter of polishing, touch the mirror as seldom as possible with the polishing-pad. Frequent polishing does far more harm than good, and the mirror, if kept carefully covered when not in use, does not need it. A fold of cotton-wool between the cap and the mirror will, if occasionally taken out and dried, help greatly to preserve the film.