A two-inch aperture, with powers of from 60 to 100, should exhibit [alpha] Piscium (3".5). | [delta] Cassiopeiae (9".5), | mag. (4 and 7-1/2) [gamma] Leonis (3".2). | Polaris (18".6), mag. (2-1/2 | and 9-1/2) A four-inch, powers 80 to 120, should exhibit [xi] Ursae Majoris (2".4). | [sigma] Cassiopeiae (3".1), | mag. (6 and 8). [gamma] Ceti (2".6). | [delta] Geminorum (7".1), | mag. (4 and 9). The tests in the first column are for definition, those in the second for illumination. It will be noticed that, though in the case of Polaris the smaller aperture may be expected to show the small star of less than the 9th magnitude, a larger aperture is required to show the 8th magnitude component of [sigma] Cassiopeiae, on account of the greater closeness of this double. In favourable weather the following is a good general test of the performance of a telescope:--A star of the 3rd or 4th magnitude at a considerable elevation above the horizon should exhibit a small well defined disc, surrounded by two or three fine rings of light. A telescope should not be mounted within doors, if it can be conveniently erected on solid ground, as every movement in the house will cause the instrument to vibrate unpleasantly. Further, if the telescope is placed in a warm room, currents of cold air from without will render observed objects hazy and indistinct. In fact, Sir W. Herschel considered that a telescope should not even be erected near a house or elevation of any kind round which currents of air are likely to be produced. If a telescope is used in a room, the temperature of the room should be made as nearly equal as possible to that of the outer air. When a telescope is used out of doors a 'dew-cap,' that is, a tube of tin or pasteboard, some ten or twelve inches long, should be placed on the end of the instrument, so as to project beyond the object-glass. For glass is a good radiator of heat, so that dew falls heavily upon it, unless the radiation is in some way checked. The dew-cap does this effectually. It should be blackened within, especially if made of metal. "After use," says old Kitchener, "the telescope should be kept in a warm place long enough for any moisture on the object-glass to evaporate." If damp gets between the glasses it produces a fog (which opticians call a sweat) or even a seaweed-like vegetation, by which a valuable glass may be completely ruined.