THE ACIIROMATIZATION OF APPROXIMATELY MONO- 

 CHROMATIC INTERFERENCE FRINGES BY A HIGHLY 

 DISPERSIVE MEDIUM, AND THE CONSEQUENT IN- 

 CREASE IN THE ALLOWABLE PATH-DIFFERENCE. 



By R. W. Wood. 



Presented by C. R. Cross, October 12, 1904. Received September 6, 1904. 



The results recorded in the present paper were, for the most part, ob- 

 tained during the progress of an investigation of the dispersion of sodium 

 vapor. As I have mentioned in the previous paper, the path-difference 

 under which it is possible to obtain interference-fringes with helium (Dj) 

 light can be more than doubled by the introduction of a small amount of 

 sodium vapor into the path of one of tlie interfering beams. This de- 

 velopment of fringes far out in the system by the dispersive action of the 

 vapor is accompanied by their complete disappearance at the centre of 

 the system, where the difference of path is zero. 



In order to understand this action of the vapor we must first consider 

 briefly the conditions under which fringes may be visible. 



Suppose that we have a system of circular fringes formed with white 

 light, and consider a point just outside of the visible ring system, where 

 the illumination appears uniform. Our fringe system is built up of an 

 infinite number of colored systems which are in coincidence at the centre, 

 but which get more and more out of step as we advance out into the 

 system, owing to the fact that the " scale" on which the Newton rings are 

 formed decreases with decreasing wave-length. Let us now consider 

 in what manner fringes may be made to appear at a point where the 

 overlapping is so great as to destroy all trace of the fringes ; in other 

 words, how may achromatization be more or less completely secured. 



It appears to me that there are only two conceivable ways in which the 

 result can be obtained. If we could, by the introduction of a dispersing 

 medium, increase the diameters of the blue rings without generally affect- 

 ing the diameters of the red ones, it is obvious that we should greatly 

 increase the number of visible fringes without, however, altering their 

 distinctness at the centre of the system. 



