318 = Prof. R. W. Wood on the Dispersion of Sodium 
would probably transmit light within this region with a 
velocity indicated by the calculated indices. The question 
of selective reflexion at the surface and refusal to transmit 
the radiation will be considered presently. 
In the following table the wave-lengths are given in the 
first column, the fringe displacements in comparison with 
helium light in the second column. ‘These values, with the 
exception of. those in the extreme red, blue-violet, and ultra- 
violet, were obtained with the interferometer. 
In the third column are given the actual fringe-shifts 
which would be found for a layer of vapour 8 ems. thick 
(effective thickness 16 cms.), and in the fourth column the 
refractive indices. The indices calculated from the disper- 
sion formula are given in the fifth column, for the sake of 
comparison with the observed values. More will be said of 
them in the next section. 
;— res He = a — ne — 
} 
| Relative = Total | Refractive | Refractive 
| Fringe-shift. Shift. | Index Cbs. | Dadex Caleul. 
7500 | 2 25 1000117 | 
6310 | 4 | BS 1000197 | 
6200 6 | 75 | 1000291 | 1-000285 
6137 7 | 88 | 1000885 | 
6055 11 138 1:000523 | 1-00052 
6013 | 14 1) 198 1000658 | 
| 5977 | 20 | 250 | 1000934 | 
| 5960 | 25 | 3138 | 1°001164 | 
| 5942 | 33 413 1001532 | 
| 5916 | 60 | 750 | 1-002972 | 
| 5875 | 100 / 1250 | 0995410 | -9958 
| 5867 | 67 | 834 «=» = 996929 | 99692 
| 5858 50 | 625 ‘997711 | 
| 5850 | 40 | 500 | -998172 | 99815 
| 5843 | 33 | 413 | -998492 | 
| 5827 25 | 313 | 998862 | 
| 5807 20) | 250 | 999098. | 
| 5750 11 | 188 | -999505 
5700 | 9 | 118 999599 | 
5650 7-4 lias 999650 
5460 4 50 ‘9998294 
5400 | 36 45 ‘9998481 
5300 2-9 | 36 ‘9998807 ‘999885 
4500 | 14 175 9999508 ‘999965 
3610 109 8, 136 9999698 
3270 | ‘9 11-4 9999768 | -999987 
2260 | 7 87 | -9999877 |  -999995 
: | 
For X= the value of m is calculated to be 10000275, 
while for A=4167 it is ‘999975, the deviation being the 
same in both cases, but in opposite directions. The complete 
dispersion-curve will be found on Plate XI. Curve 2, the 
