438 Prof. R. W. Wood on the Selective Dispersion oj 



first picture by an ink mark. On some of the plates there 

 was an indication of a slight displacement of the fringes in 

 the iron line immediately to the right of the mercury line 

 even at room temperature (23°). It is very slight however, 

 certainly not more than 0*1 of a fringe width. At 35° the 

 displacement becomes very noticeable, amounting to about a 

 quarter of a fringe width. At this temperature the pressure 

 ol the mercury vapour is 0'0004 mm. or four times the 

 pressure which it has at room temperature. The length of 

 the quartz dispersion tube being ten centimetres we cau 

 definitely state that light of wave-length 0*4 of an Angstrom 

 unit longer than that of the absorption line is retarded 0*25 

 of a wave-length in traversing a column of the vapour 

 20 cm. in length (since the light traverses the tube twice). 

 It was expected that the fringe shifts would increase pro- 

 portionally with the vapour pressure, but this was found not 

 to be the case, for at a temperature of 58°, at which the 

 pressure is 0*0215 mm., or five times as great as at 35°, the 

 displacement of the fringes in this same iron line is only 0'G 

 of a fringe width, whereas if the retardation was proportional 

 to the pressure we should expect a displacement of 1*25 

 fringe widths. It was thought at first that a mistake might 

 have been made in the identification of the fringe under 

 observation, and that the displacement might be 1*6 instead 

 of 0'6, but an examination of a very large number of photo- 

 graphs taken at intermediate temperatures showed that no 

 error had been made. The displacements of the fringes in 

 the various iron lines for the vapour at different temperatures 

 were measured as carefully as possible on the dividing- 

 engine, and are given in the following table. The wave- 

 lengths are as follows : — 



Iron Line No. 1 2535-6 



Mercury Line 2536-5 



Iron Line No. 2 2536-9 



Iron Line No. 3 2539-0 



Shift in Fringe Widths. 



Temp. 



Pressure, mm. 



No. 1. 



No. 2. 



No. 3. 



32 



00031 



016 



25 





11 



O-O062 



0-25 



0-34 





45 



0086 



030 



040 



0-11 



50 



00122 



0-40 



0-55 



018 



58 



00215 



0-50 



05 



0-25 



