May 25, 1917] 



SCIENCE 



517 



Fig. 3. The Spectrum of the Great Spiral Nebula in Andromeda, photo- 

 graphed by Slipher at the Lowell Observatory. 



[The series of bright lines along the upper and lower margins are the 

 reference spectrum. The nebular spectrum runs horizontally through the 

 central area.] 



ponderance of stars visible in the Milky 

 Way is due to the greater extension of the 

 stellar system in that direction than in the 

 direction of the galactic poles. 



Sir John Herschel, the son, extended the 

 father's search for nebulse to the southern 

 sky, by observing at the Cape of Good Hope 

 in the 1830 's. He later charted all of the 

 known nebulfe, both north and south, upon 

 a sphere representing the entire sky, and 

 found the surprising condition that the 

 nebulffi in general avoid the Milkj^ Way. 

 Several decades earlier William Herschel 

 had noticed within the Galaxy that the 

 nebulffi are the more plentiful where the 

 stars are scarce. When the stars in the 

 eyepiece of his telescope would suddenly 

 change from numerous to few he was ac- 

 customed to say to his recording assistant, 

 "Get ready, nebulse are coming." These 

 general facts of stellar and nebular distri- 

 bution, where stars are scarce nebulfe 

 abound, and where stars abound nebulffi 

 are scarce, led Herbert Spencer, among 

 othei's, to emphasize the view that the evi- 

 dence for a relationship of stars and nebulse 

 is overwhelmingijr strong. This he called 

 "the relationship of avoidance." 



In the year 1864 occurred a great astro- 

 nomical event. William Huggins pointed 

 his spectroscope to a well-known planetary 

 nebula in Draco (Fig. 1, c) and found 

 that its visible spectrum consisted of three 

 isolated bright lines (see Fig 32, N. G. C. 

 6543, the three lines at the right end of 



spectrum). This observation gave a death 

 blow to the hypothesis then prevailing that 

 all the nebulffi would prove to be clusters 

 of stars if only our telescopes were power- 

 ful enough, or if the nebulffi were brought 

 near enough to us. The spectroscope said 

 verj^ definitely and with finality : the Draco 

 nebula is unresolvable ; it is a mass of glow- 

 ing gases. A cluster of stars can not give 

 that type of spectrum. Other nebulte were 

 tested by Huggins 's spectroscope. Some of 

 these objects gave bright-line spectra, but 

 the great majority had continuous spectra. 

 Whether the latter were actually continu- 

 ous or, as in the case of the sun and other 

 middle-aged stars, the apparently continu- 

 ous spectra of the nebulffi were really inter- 

 rupted by hundreds and thousands of ab- 

 sorption lines, could not be decided because 

 the nebular spectra were exceedingly faint. 

 The eye could not have seen the absorption 

 lines even if they were present. It is only 

 in the last two decades, through the use of 

 rapid photographic plates and of exposures 

 a great many hours in length, that the 

 existence of absorption lines in the continu- 

 ous spectra of the nebulffi (see Fig. 3) has 

 been proved for all of the nebulffi sub- 

 mitted to adequate test. 



Lord Rosse's famous six-foot reflecting 

 telescope marked an epoch in nebular re- 

 search, in the year 1845, by showing that 

 certain well-known nebulffi are of spiral 

 structure — pretty certain evidence that 

 they are in rapid rotation. Roberts's 



