Glasses which are to be used in the field, should have plainly marked upon one of the eye-glass slides a focus mark, so that they may be adjusted at any time without an especial adjustment in every case. Telescopes, as a general rule, absorb light in proportion as their magnifying power is great. The most powerful glasses are therefore to be used for minute observations on the clearest days, or where there is a strong light upon the observed object. When the light is fading, or there is little light upon the observed object, the clearer view will be had by those glasses of low magnifying power. When telescopes are fitted with a double adjusting focus, the short focus is to be used, where the light is dim, the long focus where the light is strong.
Old newspapers furnish the best material with which to clean lenses.
The pieces to be selected, should be free from grit or anything to scratch the glass. Soft paper is better than chamois skin. The telescope glasses ought to be kept scrupulously clean. If the glass is to be carried in the rain, a leather cap must always cover the eye-piece end. Without this precaution, the glass will be filled with water, and may be ruined.