THE SUN’S COHONA. 
381 
the phenomenon ; others regard our atmosphere as alone in 
question ; while yet others consider that, while the illumina- 
tion of our atmosphere is in question, the moon is concerned 
in causing that atmosphere to be illuminated. 
As regards the purely optical theory, according to which the 
moon’s action on the solar rays is alone concerned, it is to be 
remarked that the only form of the theory which has been held 
worthy of much attention is that first propounded by De Lisle, 
according to which the corona is due to the diffraction of the 
solar rays passing near the moon’s edge. He allowed the sun’s 
light to pass through a small aperture into a dark room, and 
received the cone of light thus formed on an opaque disc 
somewhat larger than the cross-section of the cone. When 
this disc was viewed from behind, a margin of light somewhat 
resembling the corona could be discerned. 
We owe to the late Professor Baden Powell and to Sir 
David Brewster the thorough investigation of this theory. In 
the first place, it was pointed out that no experiments re- 
sembling those of De Lisle could establish anything respecting 
the sun’s corona, for the simple reason that behind the disc — 
that is, in the space corresponding to the region beyond the 
moon- — there was air as well as in front — a condition, of course, 
wholly distinct from that actually prevailing beyond the moon. 
Researches applied to the actual appearance of rings of light, 
formed under various conditions, led Sir David Brewster to 
enquire what relation exists between the apparent magnitude 
of such artificial coronas and the diameter of the diffracting 
disc. He found that “in all experiments of the kind the 
breadth of the ring is totally independent of the magnitude 
of the diffracting body ; ” “ therefore,” as Professor Grrant 
remarks, “ the unavoidable conclusion is, that in the case of 
the natural eclipse the ring would be utterly invisible, on 
account of the comparatively immense distance of the moon 
from the earth.” He adds, that “ this must be considered as 
a fatal objection to De Lisle’s explanation, if the principle 
upon which it is founded be admitted to be true. But, besides, 
the diffraction theory is incapable of offering any account 
whatever of many of the subordinate features of the ring, and 
therefore, upon this ground alone, it cannot be considered 
as affording a faithful representation of the phenomenon.” 
The theory which calls in the aid of the earth’s atmosphere 
presents, as I have said, two distinct forms. 
In the first place we have the theory, pure and simple, that 
the solar rays, by illuminating the upper regions of the air 
during total eclipse, cause the appearance of the corona. It is 
undoubtedly in this form that the atmospheric-glare theory 
was first presented, and to this form of the theory the argu- 
