2l8 CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 



nebulous in texture in smaller telescopes and with lower powers, 

 were resolved into stars with larger instruments and higher 

 powers. From this he argued that all the nebulae could be 

 resolved into stars by the application of sufficient magnifying 

 power, and that the nebulae were, in effect, separate universes, 

 a theory which had been earlier suggested on purely hypothetical 

 or philosophical grounds, by Wright, Lambert, and Kant. From 

 their appearance in the telescope he, again with almost uncanny 

 prescience, excepted a few as definitely gaseous and irresolvable. 



This view held sway for many years; then came the results 

 of spectroscopic analysis showing that many nebulae (those 

 which we now classify as diffuse or planetary) are of gaseous 

 constitution and can not be resolved into stars. The spiral 

 nebulae, although showing a different type of spectrum, were 

 in most theories tacitly included with the known gaseous nebulae. 



We have now, as far as the spiral nebulae are concerned, come 

 back to the standpoint of Herschel's fortunate, though not fully 

 warranted deduction, and the theory to which much recent 

 evidence is pointing, is that these beautiful objects are separate 

 galaxies, or "island universes," to employ the expressive and 

 appropriate phase coined by Humboldt. 



By means of direct observations on the nearer and brighter 

 stars, and by the apphcation of statistical methods to large 

 groups of the fainter or more remote stars, the galaxy of stars 

 which forms our own stellar universe is believed to comprise 

 perhaps a bilHon suns. Our sun, a relatively inconspicuous 

 unit, is situated near the center of figure of this galaxy. This 

 galaxy is not even approximately spherical in contour, but 

 shaped hke a lens or thin watch; the actual dimensions are 

 highly uncertain; Newcomb's estimate that this galactic disk 

 is about 3,000 Hght-years in thickness, and 30,000 light-years 

 in diameter, is perhaps as reliable as any other. 



Of the three classes of nebulae observed, two, the diffuse 

 nebulosities and the planetary nebulae, are typically a galactic 

 phenomenon as regards their apparent distribution in space, 

 and are rarely found at any distance from the plane of our 

 Milky Way. With the exception of certain diffuse nebulosities 



