Source : Webster's Revised Unabridged Dictionary (1913)

Spinthariscope \Spin*thar"i*scope\, n. [Gr. spinqari`s spark +
   -scope.]
   A small instrument containing a minute particle of a radium
   compound mounted in front of a fluorescent screen and viewed
   with magnifying lenses. The tiny flashes produced by the
   continual bombardment of the screen by the [alpha] rays are
   thus rendered visible. -- {Spin*thar`i*scop"ic}, a.