gistration of sun-spots,[412] and he enforced the suggestion,
with more immediate prospect of success, in 1854.[413] The art of
celestial photography, however, was even then in a purely tentative
stage, and Carrington wisely resolved to waste no time on dubious
experiments, but employ the means of registration and measurement
actually at his command. These were very simple, yet very effective. To
the "helioscope" employed by Father Scheiner[414] two centuries and a
quarter earlier, a species of micrometer was added. The image of the sun
was projected upon a screen by means of a firmly-clamped telescope, in
the focus of which were placed two cross-wires forming angles of 45 deg.
with the meridian. The six instants were then carefully noted at which
these were met by the edges of the disc as it traversed the screen, and
by the nucleus of the spot to be measured.[415] A short process of
calculation then gave the exact position of the spot as referred to the
sun's centre.

From a series of 5,290 observations made in this way, together with a
great number of accurate drawings, Carrington derived conclusions of
great importance on each of the three points which he had proposed to
himself to investigate. These were: the law of the sun's rotation, the
existence and direction of systematic currents, and the distribution of
spots on the solar surface.

Grave discrepancies were early perceived to exist between determinations
of the sun's rotation by different observers. Galileo, with "comfortable
generality," estimated the period at "about a lunar month";[416]
Scheiner, at twenty-seven days.[417] Cassini, in 1678, made it 25.58;
Delambre, in 1775, no more than twenty-five days. Later inquiries
brought these divergences within no more tolerable limits. Laugier's
result of 25.34 days--obtained in 1841--enjoyed the highest credit, yet
it differed widely in one direction from that of Boehm (1852), giving
25.52 days, and in the other from that of Kysaeus (1846), giving 25.09
days. Now the cause of these variations was really obvious from the
first, although for a long time strangely overlooked. Scheiner pointed
out in 1630 that different spots gave different periods, adding the
significant remark that one at a distance from the solar equator
revolved more slowly than those nearer to it.[418] But the hint was
wasted. For upwards of two centuries ideas on the subject were either
retrograde or stationary. What were called the "proper mot