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"What colour d'you want?" asked the sergeant. "What d'you mean?" "What colour d'you want?" "What choice is there?" "Ferro-prussiate or ferro-gallic." "Eh?" "D'you want ferro-prussiate or ferro-gallic?" "What's the difference?" "Brown lines or white on blue?" "I think it had better be the ferro-whatsit, white on blue." And he departed, a little wiser and somewhat chastened.
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.
We had two types of print paper, one a standard blueprint and the other a brown line; these were called in army parlance ferro-prussiate and ferro-gallic respectively. Insignias of rank do a lot to inflate egos and the Company Office WO came in full of his importance and the superior status of clerks, he demanded immediate attention and three prints of a particular drawing.
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.
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.
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