Septembee 5, 1902.] 



SCIENCE. 



371 



infoi'matiou is most important for the pur- 

 pose of calculating the orbit or path in 

 space of the star examined. 



Then, too, the same principle gives us the 

 power to measure the rotation times of the 

 sun and planets, because we can bring into 

 view two opposite sides of the sun or the 

 planet's disk; these opposite sides revolve, 

 one toward us, the other away, and the 

 spectra of the two sides show displacements 

 of lines in opposite directions. The amount 

 of displacement gives the velocity of rota- 

 tion. For the sun and planets these results 

 obtained by the spectroscope are checked 

 by independent observations, such as watch- 

 ing the spots on the sun and on Jupiter. 



The power of the spectroscope to measure 

 motion in the line of sight has recently been 

 used by the late director of Lick Observa- 

 tory, California, Keeler, and by the present 

 director, Dr. Campbell, in solving two most 

 interesting problems. When Saturn 's rings 

 were first discussed it was thought they 

 were solid. Then it was shown that a ring 

 system nearly 170,000 miles in diameter and 

 about 100 miles in thickness could not en- 

 dure, without destruction, the diverse pull- 

 ings due to the gravitation of forces exer- 

 cised by the planet and the satellites. A 

 fluid system of rings was found to be un- 

 stable also, and the theory was adopted that 

 the rings are composed of millions of small 

 satellites so aggregated that they reflected 

 sunlight to us and gave the appearance of 

 solidity, like a cloud in the summer sky. 

 This theory of the structure of the rings 

 was styled the meteoric theory: it rested 

 almost entirely on the mathematical argu- 

 ment. But Keeler in 1895 confirmed this 

 theory in a beautiful manner by the use of 

 his spectroscope. The slit of the spectro- 

 scope was made to pass through the center 

 of the image of the planet and through the 

 rings, and he obtained a photograph of 

 the spectra of the rings and the planet. 

 Then, on examination, the lines in the spec- 



tra were found to be conspicuously inclined, 

 and inclined in such a way that the planet 

 was shown to be revolving as a solid body, 

 while the rings were revolving only as they 

 could revolve if composed of separated 

 satellites. Thus we have the final proof 

 that the rings are neither solid nor liquid, 

 but are meteoric. Keeler 's results have been 

 confirmed fully by other observers. 



The question has often been asked, 'Does 

 the solar system as a unit remain fixed in 

 space, or is it moving in a known direc- 

 tion ? ' How can this be determined ? When 

 we look down a long straight line of rail- 

 road track we note that the separate tracks 

 appear to come closer together as the dis- 

 tance sighted becomes greater, and if the 

 distance is long enough the tracks appear 

 to actually come together. Now if we walk 

 down the track we discover that this com- 

 ing-together point moves away from us — 

 the tracks open in the direction we are walk- 

 ing and on looking back the tracks appear to 

 be closing in. An efirect similar to this would 

 show itself when we look at the stars, if the 

 solar system is moving in space. Those 

 stars, situated at the point towards which 

 we are moving, will gradually open out', 

 separate, and those stars in the opposite 

 direction would appear to be coming to- 

 gether. 



Observations have been made to deter- 

 mine these directions; with the result that 

 they seem to show that the sun, carrying 

 with him his family of planets, is moving 

 towards a point near the eastern edge of the 

 constellation of Hercules, with a velocity of 

 about fifteen miles a second. But observers 

 difl'er quite a little in their results. Camp- 

 bell has undertaken to investigate the sub- 

 ject by studying the velocity of stars in the 

 line of sight by the use of the spectroscope. 

 The examination of many himdreds of stars 

 ought to bring out the result, that in the 

 direction we are moving the general aver- 

 age displacement of lines in the spectra 



