tainly a remarkable fact that HERSCHEL was the first observer
to recognize the real importance of the aperture or diameter of a
telescope. Before his time it was generally assumed that this element
only conditioned the amount of light transmitted to the eye, or, in
other words, merely determined the brightness of the image. Hence the
conclusion that if an object is sufficiently bright, the telescope may
be made as small as desired without loss of power. Thus, in observing
the sun, astronomers before HERSCHEL had been accustomed to reduce the
aperture of their telescopes, in order to moderate the heat and light
transmitted. SCHEINER, it is true, nearly two centuries before the time
we are considering, had invented a method for observing the sun without
danger, still employing the full aperture. This was by projecting the
image of the sun on a white screen beyond the eye-piece, the telescope
being slightly lengthened. For special purposes this ingenious method
has even been found useful in modern times, though for sharpness of
definition it bears much the same relation to the more usual manner of
observing, that a photographic picture does to direct vision.

Although HERSCHEL saw the advantages of using the whole aperture of a
telescope in such observations, the practical difficulties in the way
were very great. We have noted his attempts to find screens which would
effectively cut off a large portion of the heat and light without
impairing vision, and have considered, somewhat in detail, the
remarkable discoveries in radiant heat to which these attempts led him.
His efforts were not unsuccessful. A green glass smoked, and a glass
cell containing a solution of black writing ink in water--were found to
work admirably.

Thus provided with more powerful instrumental means than had ever been
applied to the purpose, HERSCHEL turned his attention to the sun. In a
very short time he exhausted nearly all there was to be discovered, so
that since the publication of his last paper on this subject, in 1801,
until the present time, there has been but a single telescopic
phenomenon, connected with the physical appearance of the sun, which was
unknown to HERSCHEL. That phenomenon is the frequent occurrence of a
darker central shade or kernel in large spots, discovered by DAWES about
1858.

HERSCHEL, though observing a hundred and ninety years after the earliest
discovery of sun spots, seems to have been the first to suspect their
pe