104 PLEASANT WA YS IN SCIENCE, 



If Young had been measuring the motion of the same 

 matter which is observed with the telescope, there could of 

 course be no doubt that the telescope was right and the 

 spectroscope wrong. We might add a few yards per second 

 for the probably greater distance of the sun resulting from 

 recent transit observations. For of course with an increase 

 in our estimate of the sun's distance there comes an increase 

 in our estimate of the sun's dimensions, and of the velocity 

 of the rotational motion of his surface. But only about 

 1 2 yards per second could be allowed on this account ; 

 the rest would have to be regarded as an error due to the 

 difficulties involved in the spectroscopic method. In reality, 

 however, the telescopist and the spectroscopist observe 

 different things in determining by their respective methods 

 the sun's motion of rotation. The former observes the 

 motion of the spots belonging to the sun's visible surface ; 

 the latter observes the motion of the glowing vapours out- 

 side that surface, for it is from these vapours, not from the 

 surface of the sun, that the dark lines of the spectrum 

 proceed. Now so confident is Professor Young of the 

 accuracy of his spectroscopic observations, that he is 

 prepared to regard the seeming difference of velocity 

 between the atmosphere and surface of the sun as real. 

 He believes that "the solar atmosphere really sweeps 

 forward over the underlying surface, in the same way that 

 the equatorial regions outstrip the other parts of the sun's 

 surface." This inference, important and interesting in itself, 

 is far more important in what it involves. For if we can 

 accept it, it follows that the spectroscopic method of 

 measuring the velocity of motions in the line of sight 

 is competent, under favourable conditions, to obtain results 

 accurate within a few hundred yards per second, or 10 or 

 12 miles per minute. If this shall really prove to be 

 true for the method now, less than ten years after it 

 was first successfully applied, what may we not hope from 

 the method in future years? Spectroscopic analysis itself 

 is in its infancy, and this method is but a recent application 



