although all leave the star at the same time.  As the entire time may amount to several centuries, an excessively small difference in velocity would be recognizable.  A more delicate test would be to measure the intensity of different portions of the spectrum at a time when the light is changing most rapidly.  The effect should be opposite according as the light is increasing or diminishing.  It should also show itself in the measures of all spectroscopic binaries.

A third method of great promise depends on a remarkable investigation carried on in the physical laboratory of the Case School of Applied Science.  According to the undulatory theory of light, all space is filled with a medium called ether, like air, but as much more tenuous than air as air is more tenuous than the densest metals.  As the earth is moving through space at the rate of several miles a second, we should expect to feel a breeze as we rush through the ether, like that of the air when in an automobile we are moving with but one thousandth part of this velocity.  The problem is one of the greatest delicacy, but a former officer of the Case School, one of the most eminent of living physicists, devised a method of solving it.  The extraordinary result was reached that no breeze was perceptible.  This result appeared to be so improbable that it has been tested again and again, but every time, the more delicate the instrument employed, the more certainly is the law established.  If we could determine our motion with reference to the ether, we should have a fixed line of reference to which all other motions could be referred.  This would give us a line of ever-increasing length from which to measure stellar distances.

Still another method depends on the motion of the sun in space.  There is some evidence that this motion is not straight, but along a curved line.  We see the stars, not as they are now, but as they were when the light left them.  In the case of the distant stars this may have occurred centuries ago.  Accordingly, if we measure the motion of the sun from them, and from near stars, a comparison with its actual motion will give us a clue to their distances.  Unfortunately, all the stars appear to have large motions whose law we do not know, and therefore we have no definite starting point unless we can refer all to the ether which may be assumed to be at rest.

If the views expressed to you this morning are correct, we may expect that the future of astronomy will take the following form:  There will be at least one very large observatory employing one or two hundred assistants, and maintaining three stations.  Two of these will be observing stations, one in the western part of the United States, not far from latitude +30 deg., the other similarly situated in the southern hemisphere, probably in South Africa, in latitude -30 deg..  The locations will be selected wholly from their climatic conditions.  They will be moderately high, from five to ten thousand feet, and in desert