166 Prof. R W. Wood. 



If we calculate the refractive indices from the data given by the 

 curve traced with the diamond point, assuming the equivalent angle 

 of the stratified vapour to be 130, we get for the wave-lengths adjacent 

 to the absorption band the values //. = T0024 and //, = 0'9969. 



The value given by Becquerel (1 -0009) is intermediate between the 

 values obtained by the two methods, but I feel sure that his flame 

 must have been non-homogeneous, and his numerical results more or 

 less inaccurate in consequence. 



For the exhibition of the actual spectrum produced by a prism of 

 sodium vapour a long dispersion tube with a battery of four of five 

 prisms gives the best results. A small Bunsen burner should be used 

 for each of the fragments of sodium, which should be at least 6 or 

 8 cm. apart. (This arrangement is shown in fig. 2, p. 160.) 



The coloured drawing of the spectrum (Plate 1), to which I have 

 already alluded, was made when the spectrum was obtained in this 

 manner. A single prism gives a very pretty anomalous spectrum, 

 but the magnificent effect produced by the battery makes the slight 

 amount of extra trouble well worth while. If the electric arc is 

 employed as the source of light extreme violet will be found to occupy 

 the position of the un deviated image of the slit. Then comes the 

 blue, sometimes in contact with the violet and sometimes slightly 

 separated by a fine dark line, owing to the fact that the violet light 

 comes from the fluted carbon band of the arc, which is separated from 

 the bhie by a comparatively dark region. Then comes a wide gap 

 corresponding to light absorbed by the sodium vapour in the blue- 

 green region (the channelled spectrum), and above this a beautiful 

 flare of colour ranging from blue-green through grass-green to yellow. 

 The red and orange portion of the spectrum is on the other side of or 

 below the undeviated image, forming another brilliant flare of colour. 

 It is separated from the violet by a wide dark band, due to the 

 absorption in the vicinity of the D lines. If the density of the vapour 

 is increased by heating the tube to a higher temperature the red flare 

 extends lower down, grows fainter, and finally fades away owing to 

 the presence of the fluted absorption bands in the red. The green 

 and blue persist, however, becoming more widely separated, but finally 

 the green disappears almost entirely. It is best to arrange the gas- 

 cock so that the height of the flames can be controlled without leaving 

 the eye-piece, for it is surprising how slight a change is necessary to 

 completely alter the general appearance of the spectrum. 



The glass tube should not be allowed to cool until the experiment 

 is at an end, otherwise it will immediately fly to pieces as soon as the 

 flame is again applied to it. 



While glass answers very well if the experiment is to be of short 

 duration, sheet iron is much more satisfactory. Suitable tubes can be 

 made by any tinsmith. They should be made of thin sheet iron, and 



