[sdiy] Five-terminal Vactrol question

[Alex C. Reed IV]

I'm looking at a VTL5C3/2 5-terminal vactrol (http:// 
users.ece.gatech.edu/~lanterma/sdiy/datasheets/vactrol/vtl5cx2.pdf)  
but I am confused.  I know the two pins that correspond to the  
cathode and anode of the diode, but the other three confuse me.  Is  
there a fixed resistance between the outside pins (10MOhm, 3 and 5,  
respectively) and the middle pin sweeps between the other terminals  
(light-dependent potentiometer) or does the middle pin just give half  
the resistance across the outside pins?

Thanks for any help, and sorry if the question is dumb.  I've always  
made my own LDR cells (flat diode, light dependent resistor, and lots  
of electrical tape).

-Alex Reed