THE ECHELON SPECTROSCOPE. 

 By a. a. Michelson. 



Keceived and Presented October 11, 1899. 



The important discovery of Zeeman of the influence of a magnetic 

 field upon the radiations of an approximately homogeneous source shows 

 more clearly than any other fact the great advantage of the highest at- 

 tainable dispersion -and resolving power in the spectroscopes employed 

 in such observations. 



If we consider that in the great majority of cases the separation of 

 the component lines produced by the magnetic field is of the order of a 

 twentieth to a fiftieth of the distance between the sodium lines, it will be 

 readily admitted that, if the structure of the components themselves is 

 more or less complex, such structure would not be revealed by the most 

 powerful spectroscopes of the ordinary type. 



In the case of the grating spectroscope, besides the difficulty of obtain- 

 ing sufficient resolving power, the intensity is so feeble that only the 

 brighter spectral lines can be observed, and even these must be aug- 

 mented by using powerful discharges, — which usually have the effect of 

 masking the structure to be investigated. 



Some years ago I published a paper describing a method of analysis 

 of approximately homogeneous radiations which depends upon the 

 observation of the clearness of interference fringes produced by these 

 radiations. A curve was drawn showing the change in clearness with 

 increase in the difference of path of the two interfering pencils of light; 

 and it was shown that there is a fixed relation between such a " visi- 

 bility curve " and the distribution of light in the corresponding spectrum, 

 — at least in the case of symmetrical lines.* 



It is precisely in the examinations of such minute variations as are 

 observed in the Zeeman effect that the advantages of this method 

 appear, for the observations are entirely free from instrumental errors ; 

 there is practically no limit to the resolving power, and there is plenty 

 of light. 



* In the case of asymmetrical lines anotlier relation is necessary, and such is 

 furnished by what may be called the "phase curve." 



