2 THE CHEMISTRY OF THE SUN. [CHAP. 



the telescope is supreme ; another molecular, and here the 

 spectroscope is our guide and helper in a region full of marvels. 



What I have first to do, then, is to show on what principles 

 our inquiry in the new field of research to which I have alluded 

 and of which this book treats, is based ; how it comes that the 

 spectroscope has taken its place among the most important 

 instruments employed in scientific work ; what special qualities 

 of light it utilizes and how it utilizes them. 



And first as to the nature of light itself. In dealing with the 

 phenomena of light, or to put it in broader terms, with the total 

 radiant energy given out by the sun, or, indeed, by any other light 

 source, there is an important consideration to be borne in mind 

 in the first instance. 



The actions involved in sending a telegraphic message may 

 help us to form a correct mental image of what goes on when the 

 sensation of light is produced. We have first of all a sending 

 instrument, next a medium a wire along which the message 

 travels and finally a receiving instrument. So also in the case 

 of light. 



Dealing first with the light source the analogue of the sending 

 instrument experiment and analogy have abundantly proved 

 that that which we call light has its origin in molecules of matter 

 in a state of agitation or vibration, and the quality of the light 

 radiated depends upon the inherent nature of the molecule and 

 upon the quality of the energy which sets it in vibration or 

 controls its vibration. 



Among our most familiar light sources are the sun, the moon, 

 the stars, gas and candles ; and it will be well to remark here, 

 and the reason why will be shown in the sequel, that the light 

 which all these sources give to us is white light in the main. 



Next as to the medium. To account for the transmission of 

 light from the source to the receiver, say the eye of the observer, 

 physicists have to assume the existence of an all-pervading im- 

 ponderable fluid inappreciable to any of our senses. This they 



