Michelson A. A. Light waves and their uses (1903)

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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

Interference Methods in Astronomy 183

the interference fringes, we should have a means of making an independent measurement of the size of objects which are practically beyond the power of resolution of the most powerful telescope. The principal object of this lecture is to show the feasibility of such methods of measurement. For this purpose, however, the circular fringes that we have been investigating are not very well adapted; they are not very sharply defined; there is not enough contrast between them.

However, there is a relation which can be traced oat between the clearness of the diffraction fringes and the size and shape of the object viewed.

This relation is very complex.

The result of such calculation is that the intensity is greatest at the center, whence it rapidly falls off to zero at the first dark band. It then increases to a second maximum, where it is not more than one-ninth as great as in the center. What we should have to observe, then, is the contrast between these two parts—one but one-ninth as marked as the other and confused more or less by atmospheric disturbances. In case of a rectangular aperture the intensity curve is somewhat different, in that the maxima on either side of the central band are considerably greater, so that it is somewhat easier to see the fringes. ’ In case of the rectangular aperture the fringes are parallel to the long sides of the rectangle. The appearance of the diffraction phenomenon in this case is illustrated in Fig. 95. The pattern consists of a broad central space, whose sides are parallel to the sides of the rectangular slit, and of a succession of fringes diminishing in intensity 011