174 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1915. 



While there is no doubt that an atom of an element in the earth 

 or in a star vibrates in identical fashion under the same physical 

 conditions, it is now Imown that the frequency of vibration of an 

 element is not the exact constant that was at first supposed. It is 

 altered to a slight extent by motion of the source, by change of 

 pressure, and by the application of magnetic and electric fields. The 

 apparent change of frequency of vibration with the motion of the 

 source relative to the observer has proved an invaluable method for 

 studying the motion of stars in the line of sight, while the displace- 

 ment of the lines of hydrogen in the sun has, in the hands of Prof. 

 Hale and his assistants proved of great power in throwing light on 

 some of the physical conditions that exist in that distant body. It 

 has been found that there is order and system in the great complex 

 of modes of vibration of an atom, and that many of the lines can be 

 arranged in definite series whose rates of vibration are connected by 

 simple and definite laws. It is only within the last j'ear or two that 

 we have been able to form some idea of the origin of these spectra 

 and the meaning of a spectral series. The fact that the lightest and 

 presumably the simplest atom known, viz, hydrogen, gives a very 

 complicated light spectrum was at first, and quite naturally, be- 

 lieved to indicate that the hydrogen atom must be a very complex 

 structure. We shall see later, however, that the hydrogen atom is 

 believed to have an exceedingly simple structure, and that the com- 

 plexity of the spectrum is to be ascribed rather to a complexity in 

 the laws of radiation. 



We have seen that the study of the spectrum led Maxwell to con- 

 clude not only that the atoms were identical in weight and form but 

 that they were the only permanent and indestructible units in this 

 changing world. The apparent identity of the spectrum under all 

 conditions certainly strongly supported such a view at that time. It 

 was believed that if some of the atoms were changing, it would be 

 shown by a gradual alteration of their modes of vibration, i. e., of 

 the spectrum. It was left to the beginning of this century to show 

 the fallacy in this deduction, and to bring undoubted evidence that 

 some elements at least are undergoing spontaneous transformation 

 with the appearance of new types of matter giving a new and charac- 

 teristic spectrum. This question will be discussed later in some detail. 



ELECTRONS. 



Before, however, considering the bearing of radioactive phenomena 

 on the structure of the atom I must refer to a discovery which has 

 exercised a most profound influence on the development of physics 

 in general and on our ideas of the structure of atoms. Sir William 

 Crookes long ago found that when an electric discharge was passed 



