ASTRO-PHYSICS 265 



a hypothetical celestial sphere. Their composi- 

 tion and physical condition were held to be 

 outside the range of any definite scientific investi- 

 gation ; subjects, perhaps, better fitted to the 

 romancer than to the serious student. But with 

 the advent of the spectroscope, sun and stars, 

 in a new aspect, re-entered the realm of exact 

 knowledge, and began to give up the secrets of 

 their composition and state. 



Many of the chemical elements known on 

 the earth were detected in the sun, while dark 

 lines, not corresponding with the spectrum of 

 any terrestrial substance, suggested the existence 

 of solar elements hitherto unrecognised by the 

 chemist. The spectra of the stars were found 

 to vary, some showing the presence of hydrogen 

 only, while others indicated the existence of 

 constitutions more nearly approaching that of 

 our sun. 



The structure of the nebulae, those vast, 

 vague sources of luminosity, had long been a 

 matter of speculation. Were they clusters of 

 innumerable stars, so minute and so distant 

 that the most powerful telescopes could not 

 resolve them, or were they, indeed, as their 

 name indicated, foregatherings of cloud -like, 

 light-giving vapours ? The question was settled 

 as soon as the spectroscope was turned towards 

 their light. A continuous gradation in properties 

 was found between stars and nebulae. Some 

 nebulae gave continuous spectra, indicating high 

 density and pressure at the source of radiation, 

 others gave bright lines on a dark background — 

 the spectra, not of dense suns surrounded by 

 cooler atmospheres, but of masses of glowing 



