ADVANCE OF ASTRONOMY. 211 



loss of heat by radiation. There seems then no possi- 

 bility of the star recovering itself, unless through 

 a collision with another. For it is possible that the 

 heat produced by the impact might restore them to 

 the primitive nebulous state. If the two colliding 

 bodies were solid the result might be a shattering 

 into fragments which would be projected with high 

 velocities into space; but if the stars had not cooled 

 enough to be solid, fragmentation would be less like- 

 ly, and the collision might lead to rejuvenescence. 



The establishment of stellar physics practically 

 dates from the application of the spectroscope to the 

 investigation of the composition of the sun, the plan- 

 ets, and the stars. The facts illustrate what has 

 been repeatedly true in the history of science, that 

 the application of a new instrument or method, may 

 lead to development at a rate and in a direction which 

 no one would have ventured to predict. 



SPECTRUM AXAJLYSIS. 



The spectroscope is a combination of prisms (or 

 equivalent structures such as a " diffraction- 

 grating") by means of which the various rays com- 

 posing a particular kind of light can be separated 

 out and arranged in a line, the differences of wave- 

 length showing themselves as differences of colour. 

 Thus the presence or absence of certain kinds of 

 light can be seen at a glance. The use of the instru- 

 ment in astronomy is based on the facts (1) that the 

 quality of light is not affected by distance; (2) that 

 each element when in a glowing state emits charac- 

 teristic rays of light or has a definite discontinuous 

 spectrum; and (3) on what is known as Kirchhoff's 

 law of selective absorption. Thus the spectroscope 



