( 272 ) 
nation, however, does not clash with this law; so the objection 
moved by FiSnyi is refuted. 
With our liquids we can easily realize both cases A and B together 
and at the same time. We have but to fill the vessel with a mixture 
of equal parts of both solutions, and then to introduce one drop of 
each of the original solutions; we thus obtain a region with a minimum 
of optical density. The photograph S shows on the left a drop of 
glycerine, on the right a drop of salt solution in their opposite 
optical character. f 
On several grounds the existence of vortices in the solar atmos¬ 
phere is considered probable. In the axis of a vortex the density is 
a minimum. The refraction in the region surrounding it, must be 
comparable with that in our extended drops? It is supposed that 
sun-spots are the seat of vortices with their axes more or less per¬ 
pendicular to the photosphere 1 ). Now let a spot be situated near the 
middle of the disk. Our line of sight will make only a small angle 
with the axis of the whirl; the light travels a long way through 
the non-homogeneous region, and the effect of ray-curving must be 
greater than in the case of a spherical rarefaction. 
In order to realize similar conditions by experiment, we only 
fieed to send the light vertically through our liquids, because the 
drops extend chiefly in that direction. 
The same diaphragm with ground-glass was fixed in a horizontal 
plane, and a vessel with bottom of plate-glass placed upon it. Two 
mirrors, one beneath and one above the vessel, and making angles 
of 45° with the vertical, enabled us to arrange the illuminating and 
the photographic apparatus in about the same manner as before. 
Some results are to be found on Plate III. The photographs r\ and 6 
were obtained with salt drops in glycerine. Even on the middle of 
the luminous disk may now appear very dark spots whenever the 
line of sight makes but small angles with the long, almost cylindrical 
levels of equal density. Indeed, rays emerging in the direction of the 
line of sight from points near the axis of that rarefaction, suffered 
all of them such an intense incurvation, that they must have come 
from the dark surroundings of the source of light. 
Just as in the former set of experiments the rarefaction, it situated 
far from the centre of the disk, shows always a bright rim toward 
that centre. 
With glycerine drops in salt solution it was not so easy to obtain 
plain-shaped vertical filaments, on account of the greater mobility of 
‘) Hale, Contributions from the Mount Wilson Solar Observatory Nos. 26 and 
30; Astroph. Journ. XXVIII, Sept, and Nov. 1908. 
