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would be inconceivably minute ; so that we can scarcely con- 
ceive that, above that level, any atmosphere capable of supply- 
ing an appreciable amount of light can exist, still less that any 
atmosphere can extend to the enormous distances at which 
portions of the corona have been seen to lie above the sun’s 
surface. 
It seems to follow, as an inevitable conclusion from this 
reasoning, that the corona must consist of some sort of mafter 
— discrete solid or liquid bodies, vaporous masses, or groups 
in which solid or liquid bodies are intermixed with vaporous 
masses — travelling around the sun. There appears to me ab- 
solutely no escape from this conclusion, when we merely consi- 
der the evidence against other theories ; for I can conceive no 
other theory of the corona besides this one and those others 
which we have been forced to reject. 
The negative evidence thus bringing us to this particular 
theory, however, it will be well that we should enquire how far 
we have positive evidence in its favour.* 
In the first place, let me invite attention to what Leverrier 
has demonstrated respecting the motions of Mercury. The 
secular motion of the perihelion is not such as it should be if 
the whole mass of matter within the orbit of Mercury were 
within the visible boundary of the sun. The conclusion, to 
which Leverrier has been led by this circumstance, is that a 
zone or band of small planets exists within the orbit of Mer- 
cury. A single considerable planet would produce changes in 
the motions of Mercury in his orbit, not a change such as is 
actually observed in the orbit itself. A single planet, then, is 
not in question, but many. The change, in fact, resembles 
that produced on the orbit of Mars by the family of asteroidal 
planets. 
Now we cannot neglect such evidence as this, because it is 
precisely that sort of evidence whicli lias been found most reli- 
able. Unless observation has erred in a systematic and most 
singular manner, there is a family of small bodies within the 
orbit of Mercury. What the nature of the family may be 
Leverrier’s researches do not tell us ; but, as observations 
* In ft review of my u Other Worlds ” in the Quarterly Journal of Science 
(a review with which I Imve every reason to feel satisfied, find which has 
obviously been written by n thoughtful student of science), it is urged that 
I am too nnxious to show that every point of evidence favours a theory 
of mine — though some points may bo urged as well in favour of other 
theories. I cannot too strongly express my conviction that this anxiety is 
the very essence of safe theorising. If a theory is clearly opposed by one 
single point of evidence, it must go overbonrd, even though a thousand other 
points seem to favour it. 
