326 Prof. Wood on the Achromatization of Monochromatic 
increase in the distinctness far out in the system, at the 
expense of distinctness at the centre. This is precisely what 
happens in the case which we are considering. 
It has been found in every case that the introduction of 
sodium vapour into one path of the interferometer increases 
the distinctness of the fringes in a portion of the system 
which is brought into the field of the instrument by increasing 
the length of the other path. 
We will now consider the case of the helium fringes, 
which under ordinary circumstances disappear when the 
path difference is between 1:5 and 2 cms., there being no 
recurrence of visibility by further increment of path 
difference as in the case of sodium light. We must there- 
fore regard the helium (D,) line as a single line of finite 
breadth or a close group of lines. In fig. 1 let BC represent 
Fig. 1. 
the intensity curve of the helium light, C being the edge of 
shorter wave-length. Immediately above we have a schematic 
representation of the fringe system, with its centre at A. 
Light from the side B of the D; line will produce the fringes 
indicated by the dotted line, which are farther apart than 
the fringes formed by the light of shorter wave-length 
coming from the side C of the line. There will, in addition, 
be an infinite number of other systems formed by light of 
wave-lengths intermediate between B and C which I have 
indicated*by light shading. 
Now suppose sodium vapour to be introduced into one 
path of the instrument, and the whole system shifted slightly 
to the left in consequence. Owing to the enormous dis- 
persive power of the vapour, the dotted system (longer 2’s) 
will be shifted more than the other, since the D; line lies on 
the blue side of the sodium absorption-band, and the change 
in the velocity of the light is greatest for the longest waves, 
namely, those on the B side of the line. The result of this 
dispersive action is that the fringes are brought into step at 
a point D, to the right of the centre, thrown out-ctf-step at 
