McLeod's sunshine recorder consists of a camera fixed with its axis
parallel to that of the earth, and with the lens northward. Opposite to the
lens there is placed a round-bottomed flask, silvered inside. The solar
rays reflected from this sphere pass through the lens, and act on the
sensitive surface.
[Illustration]
The construction of the instrument is illustrated by the subjoined cut, A
being a camera supported at an inclination of 56 degrees with the horizon,
and B the spherical flask silvered inside, while at D is placed the
ferro-prussiate paper destined to receive the solar impression. The dotted
line, C, may represent the direction of the central solar ray at one
particular time, and it is easy to see how the sunlight reflected from the
flask always passes through the lens. As the sun moves (apparently) in a
circle round the flask, the image formed by the lens moves round on the
sensitive paper, forming an arc of a circle.
Although it is obvious that any sensitive surface might be used in the
McLeod sunshine recorder, the inventor prefers at present to use the
ordinary ferro-prussiate paper as employed by engineers for copying
tracings, as this paper can be kept for a considerable length of time
without change, and the blue image is fixed by mere washing in water;
another advantage is the circumstance that a scale or set of datum lines
can be readily printed on the paper from an engraved block, and if the
printed papers be made to register properly in the camera, the records
obtained will show at a glance the time at which sunshine commenced and
ceased.
Instead of specially silvering a flask inside, it will be found convenient
to make use of one of the silvered globes which are sold as Christmas tree
ornaments.
The sensitive fluid for preparing the ferro-prussiate paper is made as
follows: One part by weight of ferricyanide of potassium (red prussiate) is
dissolved in eight parts of water, and one part of ammonia-citrate of iron
is added. This last addition must be made in the dark-room. A smooth-faced
paper is now floated on the liquid and allowed to dry.--_Photo. News._
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