308 Prof. R. W. Wood on the Dispersion of Sodium 
with that within say an Angstrom unit of one of the D lines, 
as an inspection of the above table indicates. 
The dispersion within the range between D, and Ds is 
shown graphically in Curve 1, Pl. XI., ordinates and abscissee 
being inside the small rectangle. 
If this curve were completed up to \=58896, at which the 
largest recorded deviation occurs, the point in question would 
be at the 76th square below the unity line. To include this 
point it would have been necessary to take the ordinates of 
the curve on about 1/4 of the scale employed. 
Visual Observations of the Dispersion in the Visible Spectrum 
by the Method of Crossed Prisms. 
As a check on the results obtained with the interferometer, 
a series of measurements of the dispersion was made by the 
method of crossed prisms. The method was identical with 
that described in my previous paper, except that the metal 
was heated in a highly exhausted tube of hard Jena glass 
instead of in a tube filled with hydrogen. These tubes can be 
very easily prepared, and give no trouble at all. They are, 
however, apt to crack if reheated, after having been allowed to 
cool. It is not necessary to have the sodium in separate 
small lumps, as the action is not the formation of a number 
of prisms of sodium vapour, but a cylinder of vapour, very 
dense along the heated floor of the tube, and decreasing to 
very nearly zero along the top. If it is desired to prepare a 
tube for purposes of demonstration, a tube of thin steel is 
preferable, as it can be used over and over again. In this 
case it is necessary to lay a thick pad of wet asbestos-paper 
along the top of the tube, since the more uniform heating 
which results from the better conductivity of the steel is 
detrimental to the formation of the non-homogeneous cylinder 
of vapour, which only takes place when the temperature 
gradient between the top and bottom of the tube is very steep. 
The plate-glass ends are fastened on with sealing-wax, a small 
glass tube being sealed in in the manner described in my 
previous paper, through which the air is exhausted. (See 
fie. 4.) 
Glass tubes were used in making the measurements, as 
there was no occasion for cooling the tube and reheating it, 
