572 
Wert URE 
[Aprit 28, 1923 

The Interferometer in Astronomy. 
By Prof. A. S. Eppincron, F.R.S. 
Sas the naked eye the stars and planets equally 
appear as points of light. A telescope magnifies 
the planets into discs, but no telescope is large enough 
to render visible the disc of a star. We can calculate 
that a lens or mirror of 20 ft. aperture would be 
needed to show us even the largest star disc; the 
construction of such an instrument, if not hopeless, 
is far distant. We have considerable knowledge as to 
the size of stars, but until recently it was all found by 
indirect calculation ; no test had made out the image 
to be other than that of a geometrical point. At the 
risk of going over familiar ground I must consider 
briefly the mode by which a telescope forms an image 
—in particular how it reproduces that detail and con- 
trast of light and darkness which betrays that we are 
looking at a disc or a double star and not a blur eman- 
ating from a single point. This optical performance 
is called resolving power. Resolving power is not 
primarily a matter of magnification but of aperture ; 
provided we use an eyepiece of reasonably high power 
the limit of resolution is determined by the size of 
aperture of the object-glass. 
To create a sharply defined image the telescope 
must not only bring light where there ought to be 
light, but it must also bring darkness where there 
ought to be darkness. The latter task is the more 
difficult. Light waves in the ether tend to spread in 
all directions, and the telescope cannot prevent indi- 
vidual wavelets from straying on to parts of the picture 
where they have no business. But it has this one 
remedy—for every trespassing wavelet it must send a 
second wavelet by a slightly longer or shorter route 
to interfere with the first, and so produce darkness. 
This is where the utility of a wide aperture arises—by 
affording a wider difference in route of the individual 
wavelets, so that those from one part of the object- 
glass may be retarded relatively to and interfere with 
those from another part. A small object-glass can 
furnish light ; it takes a big object-glass to furnish 
darkness. 
Recognising that the success of an object-glass in 
separating double stars and other feats of resolution 
depends on the production of darkness in the proper 
place by interference between the waves from different 
parts of the aperture, Michelson asked himself whether 
the ordinary circular aperture was necessarily the most 
efficient for giving the required interference. Any 
deviation from the circular shape is likely to spoil the 
definition of the image—to produce wings and fringes. 
The image will not so closely resemble the object 
viewed. But, on the other hand, we may be able to 
sharpen up the tell-tale features. It does not matter 
how. different the image-pattern may be from the 
object, provided that we are able to read the significance 
of the pattern. If we cannot reproduce a disc, let us 
try to produce something which is distinctive of a disc. 
A little reflection suggests that we ought to increase 
the resolving power by blocking out the middle of the 
object-glass and using only two extreme regions on 
one side and the other. For these regions the differ- 
1 From the presidential address delivered before the Royal Astronomical 
Society on February 9, on presenting the gold medal of the Society to 
Prof. A. A. Michelson. 
NO. 2791, VOL. III] 


ence of light-path is greatest, and the corresponding — 
they are the most — 
wavelets are the first to interfere ; 
efficient in furnishing the dark ‘contrast needed to 
outline the image properly. 
But if the middle of the object-glass is not going to 
be used, why trouble to construct it? We are led to 
the idea of using two widely separated apertures, each 
involving a comparatively small lens or mirror—after 
the pattern of a range-finder. That is much easier to 
construct than a huge lens circumscribing both aper- 
tures. 
It is one thing to detect a small planetary or nebular 
disc ; it is another thing to make a close measure of 
its diameter. It is one thing to detect the duplicity of 
a star; it is another thing to measure the separation 
of the components. Michelson’s first experiments 
were directed not towards performing feats of resolu- 
tion beyond what had previously been attained, but 
towards improving the accuracy of measurement. He 
applied his method first to measuring the diameters of 
Jupiter’s satellites—discs which are easy to detect, but 
very difficult to measure trustworthily with an ordinary 
micrometer. But it is easier to understand the applica- 
tion of the methed to measurement of double stars than 
A 

Tic. 1. 
to the diameters of discs ; and I shall therefore speak 
more particularly of the double-star problem first, 
although that is not the historical order. 
Consider light coming from a distant point and 
passing through two small apertures, A and B, the rest 
of the object-glass being blocked out (Fig. 1). From each 
aperture the light disturbance diverges in all directions, 
and our problem is to find the nature of the luminous 
pattern formed in the focal plane, this pattern con- 
stituting the image which is viewed and magnified by 
the eyepiece. At the point P (where, according to 
geometrical optics, the single point-image ought to 
appear) w 
from the two apertures have equal paths, AP and BP, 
and reinforce one another. 
have another point of full illumination, Q, ; the paths, — 
AQ, and BQ,, are unequal, but differ by exactly a 
wave-length, so that the waves again arrive in the 
right phase to reinforce one another. Similarly we 
shall have a series of points of full illumination, Qs, Qs, 
etc., where the path-difference amounts to 2, 3, etc., 
wave-lengths. Intermediately there will be points of 
darkness where the path- -difference is }, 13, 2} wave- 
lengths, and the waves arrive in opposite phase and 
we have full illumination because the waves 
A little to one side we — 
| 
| 
