The Romance of the Heavens 51 



The modern diffraction grating is a truly wonderful piece of 

 work. It contains several thousands of lines to the inch, and 

 these lines have to be spaced with the greatest accuracy. But in 

 this instrument, again, there is a considerable loss of light. 



We have said that every substance has its own distinctive 

 spectrum, and it might be thought that, when a list of the spectra 

 of different substances has been prepared, spectrum analysis 

 would become perfectly straightforward. In practice, however, 

 things are not quite so simple. The spectrum emitted by a sub- 

 stance is influenced by a variety of conditions. The pressure, 

 the temperature, the state of motion of the object we are observ- 

 ing, all make a difference, and one of the most laborious tasks of 

 the modern spectroscopist is to disentangle these effects from one 

 another. Simple as it is in its broad outlines, spectroscopy is, 

 in reality, one of the most intricate branches of modern science. 



BIBLIOGRAPHY 



(The following list of books may be useful to readers wish- 

 ing to pursue further the study of Astronomy.) 



BALL, The Story of the Heavens. 



BALL, The Story of the Sun. 



FORBES, History of Astronomy. 



HINCKS, Astronomy. 



KIPPAX, Call of the Stars. 



LOWELL, Mars and Its Canals. 



LOWELL, Evolution of Worlds. 



McKREADY, A Beginner's Star-Book. 



NEWCOMB, Popular Astronomy. 



NEWCOMB, The Stars: A Study of the Universe, 



OLCOTT, Field Book of the Stars. 



PRICE, Essence of Astronomy. 



SERVISS, Curiosities of the Skies. 



WEBB, Celestial Objects for Common Telescopes. 



YOUNG, Text-Book of General Astronomy. 



