( 270 ) 
distribution of brightness, also when the arrangement of the density 
gradients is less simple or even very irregular, we may supplement 
the inquiry by making a few simple experiments. 
Some experiments with curved rays . 
A saturated solution of common salt has at 15° the specific gravity 
1.204; just as great is the specific gravity of a mixture of 78 parts 
of pure glycerine with 22 parts of water. A drop of one of these 
liquids keeps floating in the other for a considerable time, but gra¬ 
dually dissolves. Owing to the great viscosity of the rather strong 
solution of glycerine, no fast convection currents occur in this process: 
the chlorure of sodium and the glycerine diffuse into each other. 
The index of refraction of the glycerine solution for yellow light 
being 1.44, that of the salt solution 1.38, the optical density will 
rapidly fall off in the layer where diffusion takes place. Rays may 
bend there with a radius of curvature of only a few millimeters. 
Suppose e. g. the layer in which diffusion takes place to be at a 
dn 
certain moment 0.02 cm. thick, then we get for the average value of — 
L44- 1.38 
U.02 ~~ * 
and for the radius of curvature 
< , =i =1 ir = 0 - 47cM - 
ds 
So these liquids enable us to imitate the irregular ray-curving in 
the solar atmosphere, and thereby to preserve approximately the 
same proportion between the radii of curvature of the rays on the 
one hand, and the dimensions of the source of light on the other 
hand, if we only give the latter a diameter of some centimeters. 
The experiments were made as follows. 
Against the back of a vessel of plate-glass was placed a diaphragm 
with a circular opening, covered with ground-glass. A set of large 
strongly converging lenses formed the image of an arc light of 3 
amp. at a distance of about 6 cm. behind the centre of the opening 
in the diaphragm. The ground-glass thus became a circular source of 
light of which the brightness gradually decreased from the centre 
towards the edge. 
Tjet the vessel be first filled with our solution of glycerine. We 
force a drop of salt-solution into it from a little glass tube of which 
one end had been drawn out into a long and very thin capillary 
