Page:Light waves and their uses.djvu/150

132 astronomer may decide that the star is double. This elongation can under favorable circumstances be detected even a considerable time after the diffraction rings merge into each other. If the atmospheric conditions were a little worse, such a close double would be indistinguishable from the single star, and if the stars were a little closer together, it would be practically impossible to separate them.

Fig. 94 represents the case of a triple star whose components are so close together as to be barely within the limit of resolution of the telescope. In this case the object would probably be taken as triple because its central portion is triangular. If the three stars were a little closer together, it would be impossible to say whether the object viewed were a single or a double star, or a triple star, or a circular disc.

If now, in measuring the distance between two double stars, or the diameter of a disc such as that presented by a small satellite or one of the minor planets, instead of attempting to measure what is usually called the "edge" of the disc—which, as before stated, is a very uncertain thing and varies with the observer and with atmospheric conditions—we try to find a relation between the size and shape of the object and the clearness of