Interference Methods in Astronomy 139
tween the slits to within 1 per cent, of its whole value, and so determine the width of the line source w’ith 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 the 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 w^e 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
140
Light Waves and Their Uses
them, and the light is correspondingly feeble. We may overcome this difficulty in the same way as we did in our transformation of the microscope into the interferometer, by using mirrors to change the direction of the beam of light, instead of allowing it to pass through two apertures in front of the lens.
Fig. 101 represents two arrangements by which this may be accomplished. The light falls from above upon the two
FIG. 101
mirrors a and b, which correspond to the two slits. By these mirrors we can bend the light at any angle we choose, and bring the two beams together again at as small an angle as we wish, by means of the plane-parallel plate. Thus we can make the fringes as broad as we choose. In the second diagram we have a rather more complex arrangement of mirrors, but the effect is the same. The paths of the two rays can be easily traced in the diagrams.
If we wish to observe with such an arrangement a body of the size of a small satellite, we should have to construct the instrument so that the distance between the two mirrors could be altered, because these mirrors correspond to the