To a tutored eye a great deal is visible with a comparatively low power, and practised observers strive to use magnifying powers as low as possible, so as to diminish, as far as may be, the evils arising from an untranquil atmosphere. With a power so small as 30 or 40, many exceedingly delicate details on the moon are visible to an eye that is familiar with them under higher powers. With 200 we may say that every ordinary detail will come out under favourable conditions; but when minute points of structure, mere nooks and corners as it were, are to be scrutinised, 300 may be used with advantage. Another hundred diameters almost passes the practical limit. Unless the air be not merely fine, but superfine, the details become “clothy” and tremulous; the extra points brought out by the increased power are then only caught by momentary glimpses, of which but a very few are obtained during a lengthy period of persistent scrutiny. We may set down 250 as the most useful, and 350 the utmost effective power that can be employed upon the particular work of which we are treating. Could every detail on the moon be thoroughly and reliably represented as this amount of magnification shows it, the result would leave little to be wished for.
But it may be asked by some, what is the absolute effect of such powers as those we have spoken of, in bringing the moon apparently nearer to our eyes? and what is the actual size of the smallest object visible under the most favourable circumstances? A linear mile upon the moon corresponds to an angular interval of 0·87 of a second; this refers to regions about the centre of the disc; near the circumference the foreshortening makes a difference, very great as the edge is approached. Perhaps the smallest angle that the eye can without assistance appreciate is half a minute; that is to say, an object that subtends to the eye an arc of less than a half a minute can scarcely be seen.[7] Since there are 60 seconds in a minute, it follows that we must magnify a spot a second in diameter upon the moon thirty times before we can see it; and since a second represents rather more than a mile, really about 2000 yards, on the moon, as seen from the earth, the smallest object visible with a power of 30 will be this number of yards in diameter or breadth. To see an object 200 yards across, we should require to magnify it 300 times, and this would only bring it into view as a point; 20 yards would require a power of 3000, and 1 yard 60,000 to effect the same thing. Since, as we have said, the highest practicable power with our present telescopes, and at ordinary terrestrial elevations, is 350, or for an extreme say 400, it is evident that the minutest lunar object or detail of which we can perceive as a point must measure about 150 yards: to see the form of an object, so as to discriminate whether it be round or square, it would require to be probably twice this size; for it may be safely assumed that we cannot perceive the outline of an object whose average breadth subtends a less angle than a minute.