We will now suppose that the reader wishes to purchase a telescope or an object-glass for himself, and to be able to judge of its performance. He must have the object-glass properly adjusted in its tube, and must use the highest power; that is, the smallest eye-piece, which he intends to use in the instrument. Of course he understands that in looking directly at a star or a celestial object it must appear sharp in outline and well defined. But without long practice with good instruments, this will not give him a very definite idea. If the person who selects the telescope is quite unpractised, it is possible that he can make the best test by ascertaining at what distance he can read ordinary print. To do this he should have an eye-piece magnifying about fifty times for each inch of aperture of the telescope. For instance, if his telescope is three inches clear aperture, then his eye-piece should magnify one hundred and fifty times; if the aperture is four inches, one magnifying two hundred times may be used. This magnifying power is, as a general rule, about the highest that can be advantageously used with any telescope. Supposing this magnifying power to be used, this page should be legible at a distance of four feet for every unit of magnifying power of the telescope. For example, with a power of 100, it should be legible at a distance of 400 feet; with a power of 200, at 800 feet, and so on. To put the condition into another shape: if the telescope will read the print at a distance of 150 feet for each inch of aperture with the best magnifying power, its performance is at least not very bad. If the magnifying power is less than would be given by this rule, the telescope should perform a little better; for instance, a three-inch telescope with a power of 60 should make this page legible at a distance of 300 feet, or four feet for each unit of power.