394 THE POPULAR SCIENCE MONTHLY. 



Within the last thirty years a remarkable change, long in prepara- 

 tion,* has conspicuously affected the methods and aims of astronomy ; 

 or, rather, beside the old astronomy — the astronomy of Laplace, of 

 Bessel, of Airy, Adams, and Leverrier — has grown up a younger 

 science, vigorous, inspiring, seductive, revolutionary, walking with 

 hurried or halting footsteps along paths far removed from the staid 

 courses of its predecessor. This new science concerns itself with the 

 nature of the heavenly bodies ; the elder regarded exclusively their 

 7novements. The aim of the one is description, of the oih.'QX prediction. 

 The younger science inquires what sun, moon, stars, and nebulas are 

 made of, what stores of heat they possess, what changes are in prog- 

 ress within their substance, what vicissitudes they have undergone 

 or are likely to undergo. The elder has attained its object when the 

 theory of celestial motions shows no discrepancy with fact — when the 

 calculus can be brought to agree perfectly with the telescope — when 

 the coursers of the heavens come strictly up to time, and their 

 observed places square to a hair's breadth with their predicted 

 places. 



It is evident that very different modes of investigation must be 

 employed to further such different objects ; in fact, the invention of 

 novel modes of investigation has had a prime share in bringing about 

 the change in question. Geometrical astronomy, or the astronomy of 

 position, seeks above all to measure with exactness, and is thus more 

 fundamentally interested in the accurate division and accurate center- 

 ing of circles than in the development of optical appliances. Descrip- 

 tive astronomy, on the other hand, seeks as the first condition of its 

 existence to see clearly and fully. It has no " method of least squares " 

 for making the best of bad observations — no process for eliminating 

 errors by their multiplication in opposite directions ; it is wholly de- 

 pendent for its data on the quantity and quality of the rays focused 

 by its telescopes, sifted by its spectroscopes, or printed in its photo- 

 graphic cameras. Therefore, the loss and disturbance suffered by 

 those rays in traversing our atmosphere constitute an obstacle to prog- 

 ress far more serious now than when the exact determination of places 

 was the primary and all-important task of an astronomical observer. 

 This obstacle, which no ingenuity can avail to remove, may be re- 

 duced to less formidable dimensions. It may be diminished or par- 

 tially evaded by anticipating the most detrimental part of the atmos- 

 pheric transit — by carrying our instruments upward into a finer air — 

 by meeting the light upon the mountains. 



The study of the sun's composition, and of the nature of the stu- 

 pendous processes by which his ample outflow of light and heat is kept 

 xxp and diffused through surrounding space, has in our time separated, 



* Sir W. Herschel's great undertakings, Bessel remarks (" Populare Vorlesungen," p. 

 15), " were directed rather toward a physical description of the heavens than to astron- 

 omy proper." 



