Januaey 4, 1907] 



SCIENCE: 



The subject to which your consideration 

 is invited has to deal only with radiation 

 of this third class. Eadiation which in 

 terms of the electron theory is said to be 

 due not to abrupt or discontinuous accelera- 

 tion but to periodic acceleration. 



DEFINITION OF SPEGTEOSCOPT 



The science of spectrum analysis which 

 was born, one might almost say, in Bun- 

 sen's laboratory in 1859, developed within 

 a period of four or five years into a science 

 of a totally different character, a science 

 which enabled Kirchhofl: at once to study 

 the physical structure as well as the chem- 

 ical constitution of the sun, a science which 

 permitted Huggins as early as 1864 to dis- 

 tinguish between star clusters and nebulse, 

 a science much wider than spectrum an- 

 alysis, a branch of learning which we now 

 call spectroscopy. Briefly defined, it is 

 that science which has for its object the 

 general description of radiation, including 

 the production of radiation, the analysis of 

 radiation, the registration of radiation and 

 the measurement of radiation. 



The theory of separating, recording and 

 comparing radiation is by no means simple 

 or complete. That these last three opera- 

 tions demand in practise the highest degree 

 of skill is exemplified by the work of 

 Kayleigh, Rowland, Michelson, Perot and 

 Fabry, and Hale. 



There is, however, a certain very true 

 sense in which these last three processes 

 are merely preparatory to a more profound 

 study of the first, namely, the production 

 of radiation. Prom this point of view, 

 spectroscopy hinges upon the radiant atom 

 — if there be an atom— and may be defined 

 imperfectly and narrowly perhaps as the 

 science of the radiant atom. 



More than one brilliant and partially 

 successful attempt has been made within 

 the last quarter century to establish an 

 adequate foundation for this science by 



devising what may be called a satisfactory 

 atom. But before considering any of these 

 attempts it may be well to state briefly 

 what seem to be the criteria by which any 

 such foundation is to be judged. 



Perhaps it may be fair to consider that 

 atom as most competent which will explain 

 satisfactorily the largest number of the 

 following nine facts. 



CRITERIA 



1. The fact that spectral lines are in gen- 

 eral approximately sharp. 



2. The fact that spectral lines are never 

 perfectly sharp; but always have a finite 

 physical width. 



3. The fact that certain spectral lines are 

 arranged in series and bands after the 

 manner described so perfectly by Balmer's 

 equation and its generalized forms. 



4. The fact that increase of pressure 

 causes a shift of spectral lines toward the 

 red as discovered by Humphreys and 

 Mohler. 



5. The fact that a magnetic field will 

 transform single lines into multiple polar- 

 ized lines as discovered by Zeeman. 



6. We come now to a group of phenom- 

 ena which are not easily described under a 

 single caption. I refer to phenomena such 

 as those observed by Pliicker and Hittorf, 

 when they found one and the same gas in 

 one and the same tube yielding very differ- 

 ent spectra according to the mode in which 

 the electric discharge was applied to make 

 the gas luminous. In the same category 

 doubtless belongs the extinction of air lines 

 by the insertion of self-induction into the 

 discharge circuit. Here may belong also 

 the fact studied by Lenard and others that 

 the region near the electrode of an arc gives 

 a spectrum different from the region near 

 the center of the arc ; the fact also that the 

 so-called 'spark lines' are introduced into 

 an arc by reducing the current to small 

 values, a fact first studied by Hartmann. 



