If you fire electrons, one after the other, at a double-slit, and let them hit a wall, you get light and dark bands. If you shine a light on the electrons as they pass through the double-slit to find out which of the paths they took, there's no interference pattern, just two piles. That's the Feynman double-slit. Now... What happens if, after you shine the light, you force them through ANOTHER double-slit? Does the distance between the 1st double-slit and 2nd double-slit matter?