596 PROCEEDINGS OF THE AMERICAN ACADEMY. 



A slight inclination of either of the back mirrors of the interferometer 

 increases or diminishes the scale on which the fringes are fornaed, and 

 since a similar change in the direction of the reflected rays can be effected 

 by the introduction of an acute prism, it is easy to see that, owing to the 

 dispersion of the latter, the change iu the scale will be different for the 

 diflTerent wave-lengths, more or less perfect achromatization resulting. 



The introduction of a medium into the path of one of the interfering 

 beams causes a shift of the fringe system as a whole, and if the medium 

 is dispersing the shifts will be different for the different colors. The red, 

 green, and blue fringes, which are out of step at a given point, may thus 

 be brought into coinciilence by the inequality of their respective displace- 

 ments. In this case, however, since the systems are shifted as a whole, 

 the fringes will be thrown out of step at the centre of the system, conse- 

 quently we have obtained an 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 vapor 

 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 therefore regard 

 the helium (D3) line as a single line of finite breadth or a close group of 

 lines. In Figure 1 let BC represent the intensity curve of the helium light, 

 C being the edge of shorter wave-length. Immediately above we have 

 a schematic representatation of the fringe system, with its centre at A. 

 Light from the side B of the D3 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 

 liHit shadinor. 



Now suppose sodium vapor to be introduced into one path of the in- 

 strument, and the whole system shifted slightly to the left in consequence. 

 Owing to the enormous dispersive power of the vapor, the dotted system 

 (longer A's) will be shifted more than the other, since the Dg 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 



