Phosphoroscope
   Say "Becquerel" and the average physicist immediately thinks of Henri Becquerel (1852-1909) who discovered the phenomenon of radioactivity in 1896 while investigating the phosphorescence of uranium compounds. His father, Alexandre-Edmond Becquerel (1820-1891) is best known for his work on luminescence and phosphorescence; the scientific dynasty was established by his grandfather A.C. Becquerel (1788-1878), who developed the differential galvanometer. 

   A-E Becquerel developed the phosphoro-
scope to measure the time between the excitation of the phosphorescent material and the extinction of the glow. The sample is placed between two rotating disks with a series of holes spaced at equal angles a given distance out from the center. The holes in one disk do not line up with the holes in the other disk. The crystal is excited by light coming in through one hole, and viewed by the phosphorescent light coming out of the other hole. Varying the speed of rotation makes it possible to measure the short time interval during which the phosphorescent light is emitted. 
   This example is in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. It was made by Duboscq of Paris, and is designed to be mounted on a solar microscope.

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