capacitive sensor?

Hello. I'm thinking about creating a music and video instrument. Connected to my computer, it would have the shape of an acrylic tube. I would like to find a way to track the position of the finger on the tube's length. I thought using kind of a capacitive sensor, but I want it to be invisible that's to say hidden inside the tube; so I don't think it will work without any direct touch. Is there any sensor that could have an oblong shape? and whose voltage could be modulated by the finger? maybe a translucent film like the one used for touch screens? the tube being warped by this film? I don't know if it's feasible so please let me know your opinion. Thanks!

If the tube is transparent and if you will never play in the dark you might use light sensors or even LEDs. Have a look at my "camera" experiment on how this might be done: http://blog.blinkenlight.net/experiments/measurements/led-camera/. Of course you could use more "sensors" or less pins by means of multiplexing. But I think my approach allows for a pretty simple setup.

thanks Udo, this is really interesting but I also wanted to put light sources in the tube. they will be sound responsive so tracking finger using light won't be possible unfortunately