This correspondence in the causes of luminosity of flame,
and of gases traversed by electric currents, is supported by the
similarity of the flame-phenomena in strength and color of light.
[These researches were lately described by Dr. Werner Siemens to the
Berlin Academy.]
* * * * *
A QUICK WAY TO ASCERTAIN THE FOCUS OF A LENS.
It is well known that if the size of an object be ascertained, the
distance of a lens from that object, and the size of the image depicted
in a camera by that lens, a very simple calculation will give the
focus of the lens. In compound lenses the matter is complicated by the
relative foci of its constituents and their distance apart; but these
items, in an ordinary photographic objective, would so slightly affect
the result that for all practical purposes they may be ignored.
What we propose to do--what we have indeed done--is to make two of these
terms constant in connection with a diagram, here given, so that a mere
inspection may indicate, with its aid, the focus of a lens. All that is
required in making use of it is to plant the camera perfectly upright,
and place in front of it, at exactly fifteen feet from the center of the
lens, a two foot rule, also perfectly upright and with its center
the same height from the floor as the lens, and then, after focusing
accurately with as large a diaphragm as will give sharpness, to note the
size of the image and refer it to the diagram.
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