WOOD. — ACHROMATIZATION OF INTERFERENCE FRINGES. 597 



of the line. The result of this dispersive action is that the fringes are 

 brought into step at a point D, to tlie right of the centre, thrown out of 

 step at the centre, and still more out of step to the left of the centre. If 

 we had but the two systems indicated by the solid and dotted lines, it is 

 obvious that the systems would come into step again to the left of the 

 centre, a condition which would occur if Dg consisted of two infinitely 

 narrow lines very close together. In the actual case the presence of waves 

 of lengths intermediate between those of B and C make such a recurrence 

 of visibility to the left of the centre impossible, and we have distinct fringes 

 to one side only of the original centre of the system. On increasing the 

 density of the sodium vapor, the point D of maximum visibility moves 

 further along to the right, and to keep the fringes in the field it is neces- 

 sary to turn the screw of the instrument in such a direction as to cause 



the system to move in the same _ direction as the shift due to the sodium 

 vapor. 



Now the sodium vapor accelerates the helium light, since its refractive 

 index is less than unity for light of shorter wave-length than that of D.,, 

 consequently the reduced path is less. To shift the fringes in the same 

 direction as that resulting from the shortening of the path through the 

 sodium vapor, we must lengthen the other or air-path, which is precisely 

 what was found to be the case, as I have already said. 



If the Dg line lay on the other side of the D lines, the shift would be in 

 the opposite direction, i. e. to the right, and we might at first sight expect 

 tlie point of maximum visibility to shift to the left of the centre. AVe 

 must, however, remember that in this case the change of velocity is 

 greatest for the shortest waves on the side C of the line ; consequently 

 the system indicated by the solid line will suffer the greatest displacement, 



