592 
MR. J. NORMAN LOCKYER ON THE 
of 2nd and 3rd Contacts, but the lengths of these will be gradually diminished in 
consequence of the less oblique section of them which is made by the moon as totality 
is departed from. 
If the vapours are evenly distributed in the iayers the arcs at the cusp will be 
geometrically regular, but if they are disturbed, as we have reason to believe they 
may be, the forms of the arcs may not be regular, and they may be of unequal 
brightness in different parts. 
Although irregularities of the vapours may be distributed all along the sun’s limb, 
the conditions under which they are presented to us will vary with the distance from 
the cusp, just as in the case of uniform layers. A knot near the cusp would be fully 
revealed to us, and the prismatic camera would give us the spectrum of all its parts. 
A knot a little removed from the cusp, however, will have its base covered by the 
moon’s edge, and only the spectrum of its higher parts will be seen. Taking the 
spectrum generally, it will have four origins :—(1) the spectrum of the vapours of the 
cusps, giving bright arcs; (2) the spectrum of the visible crescent of photosphere, 
consisting of a bright continuous spectrum crossed by dark arcs ; (3) the spectrum of 
the vapours surrounding the uneclipsed part of the sun, giving bright arcs ; (4) the 
spectrum of the corona, consisting of a relatively feeble continuous spectrum and a 
number of bright rings. 
VIII. The Spectrum of the Corona. 
List of Coronal Radiations. 
Except in the case of 1474 K, the coronal rings are very feeble, and their wave¬ 
lengths can, perhaps, not be depended upon to within one-tenth metre. They were 
read off from the African negatives by direct comparison with the spectrum of 
Arcturus. Possibly, owing to the great intensity of the continuous spectrum, the 
Brazilian negatives show only 1474 K in the spectrum of the corona. 
The accompanying table indicates the wave-length of the coronal radiations, and the 
number of the African photographs in which they have been detected. It is almost 
impossible to form any trustworthy estimate of the relative intensities of the rings, 
but it may be noted that the one at wave-length 3987 comes next to 1474 K in order 
of brightness 
There are indications of other extremely faint rings, the positions of which cannot 
be determined with the necessary degree of accuracy to enable a useful statement to 
be made touching their wave-lengths. 
