Interference Methods in Astronomy
distant objects, a pair of parallel wires is placed as nearly as possible upon what is usually called the edge of the disc, as shown in Fig, 91. The position of this edge varies enormously with the observer. One observer will suppose it well within the white portion; another, on the edge of the black portion. Then, too, the images vary with atmospheric conditions. In the case of an object relatively distinct there may be an error of as much as 5 to 10 per cent. In many cases we are liable to an error ^ „
FIG. 91
which may amount to 15 per
cent., while in some measurements there are errors of 20 to 30 per cent.
Suppose the object viewed were a double star. In general, the appearance would be very much like that represented in Fig. 92, except that, as before stated, in the actual
case the appearance would be troubled by “ boiling.” It will be noted that as long as the diffraction rings are well clear of each other we need not have the slightest hesitation in saying that the object viewed is a double star.
Fig. 93 represents under exactly the same conditions two points, artificial double stars, but very much closer together. In this case the diffraction rings overlap each other. It will be seen that the central spot is elongated, and the expert
Light Waves and Their Uses
astronomer may decide that the star is double. This elongation can under favorable circumstances be detected even a
_ considerable time after the
J diffraction rings merge into
each other. If the atmospheric conditions were a little worst1, 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. 9-t 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
. . J ° FIG. 94
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