Vapour in the Visible and Ultra-violet Regions. 317 
Puke: 2 : ! A ; 
formula n= 1+ = in which m is the fringe-shift and ¢ the 
=F 
length of the column (16 cms.), the + or — sign being used 
according to the direction of the shift. 
Temperature. Fringe-shift. Temperature. Hringe-shift. 
274 560 14 | 
282 584 19 | 
ol4 595 23 | ay) 
334 | 608 Beer ae 
348 628 38 
360 644 50 
376 650 ae" 
389 
396 
408 
438 
452 
474 
496 
508 
518 
540 1 
0S Oe 
eae 
oo | Ore OMG ok Wie 
eS 
fH 
(a 
fore 
(= 
E 
| 
A=546 | 
) 
Curves are given for these optically determined densities 
in Plate XI., Curve 3. 
The Refraction and Dispersion of Sodium Vapour of 
Great Density. 
A knowledge of the absolute value of the refractive index 
of the vapour and its dispersion enables us to compile a table 
of the refractive indices for all wave-lengths, for vapours of 
varying density. This has been done for the very dense 
vapour obtained by heating a vacuum-tube containing the 
metal to a temperature of 644° C. 
A column of the vapour at this temperature 8 cms, in 
length examined by transmitted light has a distinct blue 
colour, as a result of the channelled absorption-spectrum. 
The values are given in the following table, and will be 
spoken of in future as “ observed values,”’ to distinguish them 
from values calculated from the dispersion formula. It must 
be remembered that sodium vapour as dense as that with 
which we are dealing in the present case has an absorption- 
band at the D lines broad enough to completely cut out 
everything down to and even below the helium line, at least 
for all thicknesses with which it is possible to work. On 
this account we are obliged to calculate the refractive indices 
within this region from observations made with a less dense 
vapour, a method which in the present case is probably allow- 
able within certain limits. A thin enough sheet of the vapour 
