Phos-phor′o-scope.
1. A philosophical toy, the “trap to catch a sunbeam,” consisting of glass tubes arranged in a box and containing phosphorescent substances, as the sulphides of lime, strontium, barium, etc. When this is exposed to the sun's rays or to the light emitted by a gas-burner or burning magnesium, and then removed to a dark place, each tube appears to glow with light of a different color, as
red,
blue,
green, etc.
Another mode of displaying this property in certain substances is by spreading them in thin layers on a dark surface; these may be formed into any desired fanciful figures or representations of natural objects whose images may often be presented in their natural colors by a judicious selection of the phosphorescent materials.
2. An instrument devised by
Becquerel for measuring the duration of phosphorescence in different substances.
A crystal of fluor spar or other substance to be examined is placed in a cage
a between the two perforated screens
b c, which are rotated within the casing
d by a hand-crank and gearing.
The casing has two apertures on opposite sides, so arranged that the rays from an electric light behind it shall not pass through, but are admitted to the crystal once during each revolution.
The interior screens may be rotated at a high velocity, when, if the
[
1679]
substance possesses the property of retaining light for any appreciable length of time, phosphorescence will be observed as it passes the aperture opposite the eye, and by slowing the velocity of its rotation, the duration of the luminous appearance may be timed, the number of revolutions of the crank being ascertained; for example, if this be turned once in a second, and a train of three multiplying wheels and pinions with teeth in the proportion 100 × 100 × 100 to 10 × 10 × 10 be employed between it and the screens, the latter will be rotated at the rate of 1,000 times per second.
In this way it has been found that many substances previously thought non-phosphorescent are capable of retaining light for very short periods.
