To save a lot of space here, this Wikipedia article does a good job of explaining the original Apple tube and the beam-index tube;
http://en.wikipedia.org/wiki/Beam-index_tube
The photo is my Indextron on the left next to my Sony KV-5000 CRT 5" set. The stripes are obvious.
More on the Apple can be found at Chuck A's website. Lots of original Philco documentation;
http://www.myvintagetv.com/philco_apple_tube.htm
And the schematic of the Indextron is attached if you want to reverse-engineer the thing . The photocell is on the "N" board. Courtesy of Jerome Halpen. There are previous VK threads on the Indextron if you search.