We come now to three eclipses which are especially interesting as having been all carefully observed, some observers having seen all three,--the eclipses, namely, of 1842, 1851, and 1860. Unfortunately the eclipses of 1842 and 1851 occurred when the sun-spots were neither at their greatest nor at their least degree of frequency. For a maximum of sun-spots occurred in 1837, and a minimum in 1844, so that 1842 was on what may be called the descending slope of a sun-spot wave, nearer the hollow than the crest, but not very near either: again, a maximum occurred in 1848, and a minimum in 1856, so that 1851 was also on the descending slope of a sun-spot wave, rather nearer the crest than the hollow, but one may fairly say about midway between them. Still it is essential in an inquiry of this sort to consider intermediate cases. We must not only apply the _comparentia ad intellectum instantiarum convenientium_, but also the _comparentia instantiarum secundum magis ac minus_. If the existence of great solar disturbances causes the corona to be greatly enlarged, as compared with the corona seen when the sun shows no spots, we should expect to find the corona moderately enlarged only when the sun shows a considerable but not the maximum number of spots. And again, it is conceivable that we may find some noteworthy difference between the aspect of the corona when sun-spots are diminishing in number, and its aspect when they are increasing. This point seems the more to need investigation when we note that the evidence derived from eclipses occurring near the time either of maximum or of minimum solar disturbance has not been altogether satisfactory. It may be that we may find an explanation of the discrepancies we have recognised, in some distinction between the state of the corona when spots are increasing and when they are diminishing in number.