We have already said that the record of discoveries made with really large instruments is limited; but it should also be remarked that until quite recently the number of such instruments has been very small. And not always, perhaps, have the best men had the control of them. Virtually the observer himself constitutes the most important part of his telescope: it is useless having a glass of great capacity at one end of a tube, and a man of small capacity at the other. Two different observers essentially alter the character of an instrument, according to their individual skill in utilizing its powers. Large telescopes are invariably constructed for the special purpose of discovering unknown orbs and gleaning new facts from the firmament. But in attempting to carry out this design, obstacles of a grave nature confront the observer. The comparatively tranquil and sharply definite images seen in small instruments disappear, and in their places forms are presented much more brilliant and expansive, it is true, but involved in glare and subject to constant agitation, which serve to obliterate most of the details. The observer becomes conscious that what he has gained in light has been lost in definition. At times—perhaps on one occasion in fifty—this experience is different; the atmosphere has apparently assumed a state of quiescence, and objects are seen in a great telescope with the same clearness of detail as in smaller ones. It is then the observer fully realizes that his instrument, though generally ineffective, is not itself in fault, and that it would do valuable work were the normal condition of the air suitable to the exercise of its capacity.