MODERN THEORIES OF THE SPIRAL NEBULAE.^ 
By Heeer D. Curtis, 
Director, AUerihcny Oliservatory. 
In one sense that theor}' of the spiral nebulae to which many lines 
of recently obtained evidence are pointing can not be said to be a 
modern theory. There are few modern concepts which have not been 
explicitly or implicitly put forward as hypotheses or suggestions 
long before they were actually substantiated by evidence. 
The history of scientific discovery affords many instances where 
men with some strange gift of intuition have looked ahead from 
meager data, and have glimpsed or guessed truths which have been 
fully verified only after the lapse of decades or centuries. Herschel 
was such a fortunate genius. From the proper motions of a very 
few stars he determined the direction of the sun's movement nearly 
as accurately, due to a very happy selection of stars for the purpose, 
as far more elaborate modern investigations. He noticed that the 
star clusters which appeared nebulous in texture in smaller tele- 
scopes 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 b}' Wright, Lambert, and 
Kant. From their appearance in the telescope he, again with almost 
uncanny prescience, excepted a few as definitely gaseous and irre- 
solvable. 
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. 
1 Abstract of a lecture given on Mar. 15, 1919, at a joint meeting of the Washington 
Academy of Sciences and the Philosophical Society of Washington. The lecture was 
illustrated witU numerous lantern slides. Rtprinted by permission from the Journal of 
the Washington Academy of Sciences, vol. 9, No. 8, Apr, 19, 1919. 
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