94 
LUMMER. 
the radius of curvature of the refracted rays can become less 
than the radius of Neptune. On the earth the least possible 
radius of curvature of refracted rays, p, Fig. 5, is always 
greater than the radius, r, of the earth, and thus an observer 
can never see objects which are situated on the opposite side 
of the earth. 
In the opinion of Schmidt, our sun, as a gaseous sphere, 
belongs to the Neptune type. Since its radius is 108 times 
as great as the radius of the 
earth, all rays of every wave- 
length emitted by the central 
mass of gas return again to 
this central mass (like the 
ray aob in Fig. 6), as do also 
the rays which come from 
parts near the center; there- 
fore from these parts no ray 
leaves the gaseous sphere in a 
direction perpendicular to 
the diameter, MAS, of the 
sun, and no such ray reaches 
the observer, 0 , on the earth. 
Only one sphere, which we 
will call the critical sphere, is 
characterized by the radius of 
curvature, p, of tangential 
rays equal to the radius, MA, 
the sphere itself. A ray once exactly perpendicular 
to the sun’s diameter at this sphere would never leave 
it. but would run forever around its periphery. 
This critical sphere, therefore, is the exterior limit 
of those parts of the sun from which rays, LAO, emitted 
by the central mass of gas can leave in a nearly tangential 
direction and reach the observer, 0, on the earth. Rays 
emitted from more distant zones in a tangential direction, as 
RSO, have their origin not in the white incandescent central 
regions, but in exterior zones, filled with rarefied gases (the 
chromosphere) emitting line spectra. The “critical” 
(• 
OT 
zone 
