The apparatus is of simple construction. It consists of a glass sphere
silvered inside and placed before the lens of a camera, the axis of the
instrument being placed parallel to the polar axis of the earth. The
whole arrangement will be readily understood by an inspection of Fig. 1.
The light from the sun is reflected from the globe, and some of it,
passing through the lens, forms an image on a piece of prepared paper
within the camera. In consequence of the rotation of the earth, the image
describes an arc of a circle on the paper, and when the sun is obscured,
this arc is necessarily discontinuous. The image is not a point, but a
line, and in certain relative positions of the sphere, lens, and paper,
the line is radial and very thin, so that the obscuration of the sun for
only one minute is indicated by a weakening of the image.
[Illustration: FIG. 1.]
In the actual apparatus the sphere is an ordinary round-bottomed flask
about 95 mm. in diameter, and the lens a simple double convex lens of
about 90 mm. focal length. The sensitive paper employed is the ordinary
ferro-prussiate now so much used by engineers for copying tracings. This
was selected in consequence of the ease with which the impression is
fixed, for the paper merely requires to be washed in a stream of water
for six minutes, no chemicals being necessary. When the paper is dry,
radial lines containing between them angles of 15° are drawn from the
center of the circular impression, and thus give the hour scale, the time
of apparent noon being of course given by a line passing through the plan
of the meridian. Fig. 2 is a copy of the record of June 27, 1884; in the
morning the sun shone brightly, toward noon clouds began to form, and in
the afternoon the sky was hazy. The field in which the instrument is
placed is surrounded by trees, so the ends of the trace are cut off
sharply by shadows.
[Illustration: FIG. 2.]
With the alteration of declination of the sun, the light entering the
camera is reflected from different portions of the sphere, and an
alteration of the position of the focus results. This may be corrected in
three ways; by moving (1) the paper, (2) the lens, or (3) the sphere. In
the present apparatus the first method has been adopted, and now the
camera is about twice as long as it was in June. As a consequence, the
circular image is enlarged, and the light therefore weakened, and that at
a time of year when it can least be spared. If the focus is altered by
moving the lens, the winter circle is small and the summer circle is much
larger. This would perhaps be too much to the advantage of the winter
sun. If, however, the lens and paper are maintained at a constant
distance, and the sphere alone moved, the circles are more nearly of the
same diameter throughout the year, the winter one still remaining the
smallest. This seems, therefore, to be the most advantageous arrangement,
and the one that will be adopted in future. It may be possible also to
find positions for the sphere, lens, and paper such that the intensity of
the image is a true measure of the intensity of the sun's light; at
present, however, this has not been done, the want of sunlight and the
press of official work having prevented the carrying out of the necessary
experiments. A more sensitive paper might also be used with advantage,
and in observatories where photographic processes are carried on daily
there would be no difficulty on this score, but my principal object was
to devise some economical instrument requiring only easy manipulation, so
that at a considerable number of places the instruments might be set up,
giving a more useful average of the duration of sunshine than can be
obtained from only a few stations. The instrument also gives a record
when the sun is shining through light clouds; in this case the image is
somewhat blurred and naturally weakened, and it may be difficult or
impossible to employ any scale for measuring the intensity under such
conditions, but it must be remembered that, even when the sun is shining
in this imperfect manner, it is really doing work on the vegetation of
the earth, and deserves to be recorded.
It may be well to say that the instrument is in no way protected. Some
friends, whose opinion I highly value, urged me to patent it; but as I
strongly hold the view that the work of all students of science should be
given freely to the world, the apparatus was described at the Physical
Society a few hours after the advice was given, lest the greed of filthy
lucre should, on further deliberation, cause me to act contrary to my
principles.--_Herbert McLeod, Nature_.
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