82 Transactions of (he 



THE SPECTRUM. 



Of all the discoveries made within the last few years, none has 

 deserved more attention, or more admiration, than the results of 

 the application of spectrum analysis to Chemistry. We are now 

 able to decide, with positive certainty, about the existence of 

 certain elements in various bodies ; and not only so, but even to 

 mention without a doubt many of the substances contained in 

 the sun and in the stars, while the composition of those mysterious 

 nebuUe has to a great extent been revealed. But we know com- 

 paratively little to what we shall some day probably know. 

 Every year brings some new discoveries to light. Within a very 

 short time, four new elements have, by the aid of the spectrum, 

 been discovered, namely, Indium, Caesium, Rubidium, and Thal- 

 lium. When we consider the distance of the sun, and the almost 

 incredible distance of the nearest fixed star from our earth, it 

 seems more than wonderful that we should be able to learn so_ 

 much about such bodies. We must, therefore, look upon the dis- 

 covery of spectrum analysis as one of the most remarkable that 

 has ever been made, and try to make good use of it. 



Let us first examine into the appliances of the spectroscope to 

 terrestrial matters. Before doing so, however, we must inquire 

 into the nature of light and the construction of the instrument. 



There are two theories about light. The first is the Emission 

 or Corpuscular Theory, supported so strongly by Newton. The 

 second is the Undulatory Theory, which has been advocated and 

 established by Young. On the first, it is assumed that luminous 

 bodies emit, in all directions, an imponderable substance consist- 

 ing of molecules, which are propagated in right lines with an 

 almost incredible velocity. These penetrate the eye, act on the 

 retina, and determine the sensation which constitutes vision. On 

 the undulatory theory, all bodies, as well as the celestial spaces, 

 are filled with an extremely subtle elastic medium, which is called 

 the luminiferous ether. The luminosity of a body is due to an 

 infinitely rapid vibratory motion, which calls forth vibrations of 

 the ether, which are propagated in all directions, and on striking the 

 retina produce the sensation of vision. This theory is now almost 

 universally held, as it explains many phenomena which, the other 

 could not. Light, heat, and sound are the three great vibratory 

 motions ; and I shall try in this paper to point out analogies be- 

 tween them whenever an opportunity offers. 



In the year 1675, Sir Isaac Newton brought his famous treatise 

 on ' Opticks' before the Royal Society. He heads the first para- 

 graph in his memoir, ' Lights which differ in colour differ also in 

 refrangibility.' This he showed by decomposing the white light 

 of the sun. He allowed a pencil of solar light to pass through a 



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