32 THOUGHTS ON NATURAL PHILOSOPHY. 



The spectroscope is essentially an instrument 

 whereby light consisting of a mixture of colours is, 

 after entering the instrument by a narrow slit, 

 resolved into its constituent colours by a prism, or 

 occasionally by an equivalent " diffraction grating." 

 The separated colours are in either case spread out 

 into a tinted band or " spectrum." About the 

 middle of the present century observations with 

 the spectroscope had indicated that there was a 

 remarkable difference between light emitted by a 

 glowing gas and that radiated from an incandescent 

 solid or liquid body. With light emanating from an 

 incandescent solid or liquid, such as that emitted by 

 a glowing mass of white-hot metal, or by a gas flame 

 in which the greater part of the luminosity is due to 

 incandescent clouds of soot deposited in the flame 

 from the decomposition of the gas under the intense 

 heat of combustion, and, with a limitation to be 

 noticed subsequently, that from the Sun and from 

 the great majority of the stars, the spectrum consists 

 of a continuous band in which all the colours of the 

 rainbow are represented, each passing into the next 

 by insensible gradations, while red and violet occupy 

 the extreme positions. In the light from a glowing 

 gas, however, at any rate when the density of the 

 gas is not excessive, this is not the case, the light 

 being now resolved into a series of clearly-defined 

 and separate colours, which appear in the spectro- 

 scope as bright lines of coloured light separated by 

 dark intervals; the lines are, in fact, images of the 

 slit by which the light enters the instrument, a 

 separate image being formed by each of the colours 



