ii4 THE POPULAR SCIENCE MONTHLY 



same for all stars of the same class of spectrum. It could be tested by 

 the stars forming a cluster, like the Pleiades, which are doubtless all at 

 nearly the same distance from us. The spectra of stars of the tenth 

 magnitude, or fainter, can be photographed well enough to be measured 

 in this way, so that the relative distances of nearly a million stars could 

 be thus determined. 



Another method which would have a more limited application, would 

 depend on the velocity of light. It has been maintained that the veloc- 

 ity of light in space is not the same for different colors. Certain stars, 

 called Algol stars, vary in light at regular intervals when partially 

 eclipsed by the interposition of a large dark satellite. Eecent observa- 

 tions of these eclipses, through glass of different colors, show variations 

 in the time of obscuration. Apparently, some of the rays reach the 

 earth sooner than others, 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 investiga- 

 tion 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 refer- 

 ence 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 mo- 



