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This "Geigerscope" is what other people would refer to as an alphascope, similar to a spinthariscope without its own radioactive source. (Spinthariscopes are explained in more detail here.)
It has a lens at one end and a luminous screen at the other (underneath the small cap you see in the photograph). The idea is to point the screen at a radioactive object and look through the lens to see the flashes of light generated on the luminous screen. Based on the number of flashes you can estimate the activity level of the source. A scope like this is far simpler and cheaper than a Geiger counter (just a few dollars vs. Hundreds) and it requires no batteries. It basically can't fail unless you run over it with a truck. On the other hand, it works only in a dark room with dark adapted eyes, and it provides only a rough indication of activity level.
Source: eBay seller geoelectronics
Contributor: Theodore Gray
Acquired: 19 May, 2007
Text Updated: 19 May, 2007
Sample Group: Spinthariscopes