Next comes a caution which beginners specially need. Almost everyone on getting his first telescope wants to see everything as big as possible, and consequently uses the highest powers. This is an entire mistake. For a telescope of 2-1/2 inches aperture two eye-pieces, or at most three, are amply sufficient. Of these, one may be low in power, say 25 to 40, to take in large fields, and, if necessary, to serve in place of a finder. Such an eye-piece will give many star pictures of surprising beauty. Another may be of medium power, say 80, for general work; and a third may be as high as 120 for exceptionally fine nights and for work on double stars. Nominally a 2-1/2 inch, if of very fine quality, should bear on the finest nights and on stars a power of 100 to the inch, or 250. Practically it will do nothing of the sort, and on most nights the half of this power will be found rather too high. Indeed, the use of high powers is for many reasons undesirable. A certain proportion of light to size must be preserved in the image, or it will appear faint and 'clothy.' Further, increased magnifying power means also increased magnification of every tremor of the atmosphere; and with high powers the object viewed passes through the field so rapidly that constant shifting of the telescope is required, and only a brief glimpse can be obtained before the instrument has to be moved again. It is infinitely more satisfactory to see your object of a moderate size and steady than to see it much larger, but hazy, tremulous, and in rapid motion. 'In inquiring about the quality of some particular instrument,' remarks Sir Howard Grubb, 'a tyro almost invariably asks, "What is the highest power you can use?" An experienced observer will ask, "What is the lowest power with which you can do so and so?"'