tween the slits to within 1 per cent. of its whole value, and so determine the width of the line source with a corresponding degree of accuracy.

The visibility curve shown in Fig. 100 represents the case in which the source is a double disc—a double star, for instance, in which theFIG. 100 discs have appreciable magnitude. The envelope of the curve, which is drawn full, corresponds to the circular form of the separate discs, and from this curve we can determine the size of the separate discs, provided they are equal. The dotted curve tells us that we are dealing with a double object. Hence, if in observing a heavenly body we obtain a visibility curve of this form, we infer that we are dealing with a double star.

There is a difficulty in carrying out such observations, especially when we are observing a very small object or a very close double star. For in this case the slits have to be separated rather widely, and the angle between the rays from the two slits, when they come together, is rather large. Hence, the distance between the interference fringes is correspondingly small, as was shown in a previous lecture, and this distance becomes less and less as the angle becomes greater and greater. When we approach the limit of resolution of the telescope, the fringes are so small that a rather high power eyepiece must be used in order to see