Fig. 72.
The Fluoroscope.
This is a funnel-shaped cardboard box with an opening at the smaller end for the eyes and a piece of card across the larger end. The inside surface of this card is covered with crystals of barium platino cyanide, the most satisfactory fluorescent substance obtainable. The earlier fluoroscopes were made with tungstate of calcium, but the above salt has proven far more satisfactory. The operation of the fluoroscope is simple. It is held in the hand by a convenient handle, the open end pressed close to the eyes, so as to exclude outside light, and with the hand or other object held against the outside of the big end, or screen, it is directed towards the Crookes tube. The screen then appears to glow with a bluish light, and the shadow of the object is distinctly seen on the screen. Different adjustments of the coil give results which will be treated upon later.
Phosphorus Tube.
Messrs. Siemens and Halske manufactured a tube which allowed of a slight variation of vacuum by using the vapor of phosphorus. An auxiliary tube containing phosphorus was added to the main tube, and upon heat being applied to it by means of a lamp, vapor is given off, which materially reduces the vacuum of the main tube. When the opposite result is desired part of the current is diverted through the auxiliary tube, and the vapor is caused to solidify itself upon the walls of the tube.
Fig. 73.