_Powers requisite for observing the phenomena of the planets._--The planet Mercury requires a considerable magnifying power, in order to perceive its phases with distinctness. I have seldom viewed this planet with a less power than 100 and 150, with which powers its half moon, its gibbous, and its crescent phase, may be distinctly perceived. With a power of 40, 50, or even 60 times, these phases can with difficulty be seen, especially as it is generally at a low altitude, when such observations are made. The phases of _Venus_ are much more easily distinguished, especially the _crescent_ phase, which is seen to the greatest advantage about a month before and after the inferior conjunction. With a power not exceeding 25 or 30 times, this phase, at such periods, may be easily perceived. It requires, however, much higher powers to perceive distinctly the variations of the gibbous phase; and if this planet be not viewed at a considerably high altitude when in a half-moon or gibbous phase, the obscurity and undulations of the atmosphere near the horizon, prevent such phases from being accurately distinguished, even when high powers are applied. Although certain phenomena of the planets may be seen with such low powers as I have now stated, yet, in every instance, the highest magnifying powers, consistent with distinctness, should be preferred, as the eye is not then strained, and the object appears with a greater degree of magnitude and splendour. The planet _Mars_ requires a considerable degree of magnifying power, even when at its nearest distance from the earth, in order to discern its spots and its gibbous phase. I have never obtained a satisfactory view of the spots which mark the surface, and their relative position, with a less power than 130, 160, or 200 times; and even with such powers, persons not much accustomed to look through telescopes, find a difficulty in distinguishing them.