William Herschel 93 



can only indicate a motion relative to the earth, we must 

 conclude that if we consider the system of stars as a whole 

 to be at rest, our sun with its planetary system moves 

 towards a definite point in the heavens. If, on the other 

 hand, we consider the solar system to be at rest, then the 

 great majority of stars are drifting in nearly parallel 

 directions, and whatever view we may take it is certain that 

 the star velocities are not entirely independent of each 

 other. The subject is one that has received renewed 

 attention in recent years; it has now been demonstrated 

 that there is more than one star-drift, and Herschel's work 

 is likely to develop into an important -department of 

 astronomy. 



One further discovery of considerable interest and im- 

 portance but belonging to the domain of physics, remains 

 to be noted. In order to compare the heating effects 

 of the coloured rays of which, as Newton taught us, solar 

 light is composed, Herschel placed thermometers in the 

 different portions of a spectrum obtained by means of a 

 prism. He noted that the heating powers of the rays 

 continuously increased from the blue through the green 

 and yellow to the red. He then discovered that the 

 thermometer rose highest when placed outside the red, 

 proving that the solar spectrum contains invisible rays less 

 refrangible than the red. These rays, though they do not 

 affect our eye, become apparent by means of then* heating 

 effect. Herschel satisfied himself that these invisible rays 

 were refracted and reflected according to the ordinary laws. 



The idea of invisible radiations, refrangible like light at 

 the surface of transparent bodies was at that time entirely 

 novel, and must have appeared almost as surprising as the 

 discovery of Roentgen rays in our own time. The heat 

 radiations were at first looked upon with scepticism, and 

 met with opposition in some quarters, even when Wollaston 

 soon afterwards proved the existence of other rays beyond 

 the violet end of the spectrum which showed themselves by 

 their chemical effects. 



The success of experimental investigation depends so 

 much on the use of scientific instruments and appliances 

 that the important share contributed to the progress of 



