330 Achromatization of Monochromatic Interference Fringes. 
the emitting slit of the monochromatic illuminator was 
bisected with a wire which cut out the D lines from the 
narrow hand of the spectrum which was utilized. 
Note by Lord RaYLEicH. 
Having had an opportunity of seeing the above paper in 
proof, I append with Prof. Wood’s permission a few remarks. 
The remarkable shift of the bands of helium light when a 
layer of sodium vapour is interposed in the path of one of the 
interfering pencils, is of the same nature as the displacement 
of the white centre found by Airy and Stokes to follow the 
insertion of a thin plate of glass. If D denote the thickness 
of the plate and yu its refractive index, (u—1) Dis the retard- 
ation due to the insertion of the plate, and if R be the 
relative retardation due to other causes, the whole relative 
retardation is 
R+(e—1) DD, . 
in which R and D are supposed to be independent of the 
wave-length A, while ~ does depend upon it. The order of 
the band (n) is given by 
R+(u—1) D 
a , 2 
: @) 
For the achromatic band in the case of white light, or for 
the place of greatest distinctness when the bands are formed 
with light approximately homogeneous, n must be stationary 
as W varies, 1. é., 
nr 
dn 
a =O. PS i 
For a small range of wave-length we may write 
so that A=) + Or, 
R+(m,+ Sheet) D 
i ; dry 
Her wee 
R+(“,—1) D or du R+(m—1)D 
ate De he oO 
Xo Ks Ay \ dro Xo © 
The achromatic band occurs, not when the whole relative 
retardation (1) vanishes, but when 
R+ (4-1) D= Dry E "a 
If D.be great enough, there is no limit to the shift that may 
be caused by the introduction of the dispersive plate. 
