cil drawings, while ordinarily less trustworthy
because involving the uncertain element of personal equation are more
valuable in delineating the finest and faintest detail of which the
sensitive plate rarely takes note; the vast array of both, however,
shows marked differences in the structure and form of the Corona from
one eclipse to another though it has not yet revealed rapid changes
during any one observation. This last interesting feature can be studied
only by comparison of photographs near the beginning of an eclipse track
and its end, two or three hours of absolute time apart." Concerted
efforts to accomplish this were made in 1871, 1887, and 1889, but they
broke down because the weather failed at one or other end of the chain
of observing stations and a succession of photographs not simultaneous
but separated by sufficient intervals of time could not be had. The
eclipse of 1893, however, yielded successful though negative results.
Photographs in South America compared with photographs in Africa two
hours later in time disclosed no appreciable difference in the structure
of the Corona and its streamers. The eclipse of May 28, 1900, will
furnish the next favourable opportunity for a repetition of this
experiment by reason of the fact that the line of totality begins in
North America, crosses Portugal and Spain and ceases in Africa. In other
words, traverses countries eminently calculated to facilitate the
establishment of photographic observing stations where observations can
be made not simultaneously but at successive intervals spread over
several hours.

Although of course the Corona had been observed long before the year
1851, as indeed we have already seen, yet the eclipse of 1851 is the
farthest back which we can safely take as a starting-point for gathering
up thoroughly precise details, because it was the first at which
photography was brought into use. Starting, therefore, with that eclipse
I want to lay before the reader some of the very interesting and
remarkable generalisations which (thanks especially to Mr. W. H.
Wesley's skilful review of many of the photographic results) are now
gradually unfolding themselves to astronomers. To put the matter in the
fewest possible words there seems little or no doubt that according as
spots on the Sun are abundant or scarce so the Corona when visible
during an eclipse varies in appearance from one period of _eleven_ years
to another like period. Or, to put it i