CHAPTER I. 



INTRODUCTION. 



Recent Spectroscopic Investigations. Spectra of Gases, Spectra of Liquids and 

 Solids, banded Spectra. A Method of Chemical Analysis. Atomic Structure 

 and Spectra. Organic Absorption Spectra, including Unit of Absorption, The- 

 ory of Chromophores, Theory of Dynamic Isomerism, and Stark's Theory. The 

 Complexity of Spectra. Method of Attacking the Problem of Emission and 

 Absorption Spectra. 



The study of the various phenomena of light may be divided into 

 three parts: The emission of light by matter, the transmission of light 

 through space, and the absorption of light by matter. The theory of the 

 transmission of light as an electromagnetic phenomenon was first proposed 

 by Faraday and Maxwell. On this theory it is assumed that in all regions 

 of space through which light passes there are electric and magnetic fields. 

 In an electric field there exists a certain state of things that gives rise to a 

 force acting on any electric charge that may exist there. This is the electric 

 force and this represents the state of the region of space considered. In a 

 similar way the magnetic field is also defined. A relation is then found 

 between the electromagnetic quantities which is usually called Maxwell's 

 equations, or is a modified form of these equations. Starting with these 

 equations, Maxwell showed that the state of things represented by his 

 fundamental equations consists of the propagation of a periodic variation 

 of the electric and magnetic forces through space with the velocity of light. 

 So well does the nature of these electromagnetic waves agree with the prop- 

 erties of light as transmitted by the ether and transparent bodies that light 

 is at present considered to be an electromagnetic disturbance itself. The 

 simplest case of light-waves is that of plane polarized waves traveling in 

 the direction of the x axis. Waves of this kind are: 



E y = a cos n(t j H z = a cos nit j 



E y , the component of the electric force in the y direction, is the only com- 

 ponent of the electric force that has a value. The magnetic force has a 

 component only in the z direction, H z . a is the amplitude of the disturb- 

 ance, / is the time, n is the number of vibrations in a time 2k and c is the 

 velocity of light. 



RECENT SPECTROSCOPIC INVESTIGATIONS. 



A light-wave in the "ether" is an electromagnetic disturbance that is 

 propagated in free space without any distortion of form or any dissipation 

 of energy, one of the properties of electric and magnetic fields being the 

 power to store energy. When a light-wave strikes ordinary matter it is in 

 general broken up into several parts. If the surface of the body is smooth, 

 a considerable part of the energy will be taken up by a regularly reflected 



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