ve 
‘apour in the Visible and Ultra-violet Regions. 308 
A number of observations were made, the mean of which gave 
5:6 as the ratio of the shifts of wave-lengths 5770 and 5790. 
These values fitted almost exactly on the dispersion-curve 
obtained with the monochromatic illuminator and the helium 
tube. Obviously a more accurate method would have been 
to provide a device by which the light of one of the yellow 
lines could have been cut off as desired. An attempt was 
made to do this with the monochromatic illuminator, but the 
loss of light was too great. 
Some very curious results were obtained by illuminating the 
interferometer with sodium light. The fringes disappeared at 
the moment when the thermo-element indicated a temperature 
of 180°. It was at first thought that the disappearance was due 
to absorption of the light by the vapour in one of the two paths, 
but on covering up the other path it was found that the field 
of the instrument was still brightly illuminated. On turning 
the screw of the instrument the fringes appeared again, and 
a little further experimenting showed that the effect of the 
vapour was to alter the visibility curve, its periodicity be- 
coming less as the density of the vapour increased. No 
satisfactory explanation of the altered appearance of the 
fringe system could be found, and an attempt was made to 
get rid of one of the D lines in the illuminating light, thus 
simplifying the conditions. Prisms and gratings were tried, 
but the loss of light was so great, that even with an oxy- 
hydrogen sodium flame the fringes could be made out only 
with the greatest difficulty. The desired result was finally 
obtained by mounting a quartz crystal between two crossed 
nicols. A number of crystals were examined with polarized 
light perpendicular to the axis, and one selected which showed 
dark bands in the spectrum separated by a distance about 
double the distance between the D lines. The nicols were 
arranged so that the emergent light vibrated in such a plane 
as to be most copiously reflected by the interferometer mirror, 
2. . the first nicol was mounted with its short diagonal hori- 
zontal, while the one next to the instrument was placed with 
its long diagonal horizontal. Between the nicols was mounted 
the quartz crystal with its axis making an angle of 45 degrees 
with the vertical. The light from an oxy-hydrogen sodium 
flame, after passage through the polarizing system, was brought 
toa focus on a cardboard screen by means of alens, forming a 
series of bright bands separated by dark intervals. A narrow 
slit in the screen allowed light corresponding to D, or D, to 
enter the interferometer, according as it was set on one edge 
or the other of one of the bright bands. This arrangement 
worked admirably, and yielded an abundance of light. No 
