_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.
The strongest and most prominent _belts of Jupiter_, may be seen with a power of about 45; which power may be put upon a 20-inch achromatic, or a 1 foot reflector. But a satisfactory view of all the belts, and the relative positions they occupy, cannot be obtained with much lower powers than 80, 100, or 140. The most common positions of these belts are--one dark and well-defined belt to the south of Jupiter’s equator; another of nearly the same description to the north of it, and one about his north and his south polar circles. These polar belts are much more faint, and consequently not so easily distinguished as the equatorial belts. The _moons_ of this planet, in a very clear night, may sometimes be seen with a pocket 1 foot achromatic glass, magnifying about 15 or 16 times. Some people have pretended that they could see some of these satellites with their naked eye; but this is very doubtful, and it is probable that such persons mistook certain fixed stars which happened to be near Jupiter for his satellites. But, in order to have a clear and interesting view of these, powers of at least 80 or 100 times should be used. In order to perceive their immersions into the shadow of Jupiter, and the exact moment of their emersions from it, a telescope not less than a 44 inch achromatic, with a power of 150 should be employed. When these satellites are viewed through large telescopes with high magnifying powers, they appear with well defined disks, like small planets. The planet Jupiter has generally been considered as a good test by which to try telescopes for celestial purposes. When it is near the meridian and at a high altitude, if its general surface, its belts, and its margin appear distinct and well-defined, it forms a strong presumptive evidence that the instrument is a good one.