The threshold of dark-adapted (scotopic) vision is 40 pWm-2 at a central wavelength of 500 nanometer. (Note: 1pW is 1 x 10-12 W.) If light of this intensity and wavelength falls perpendicularly on an eye with the pupil open to its maximum diameter of 8 millimeter, find how many photons enter the pupil in one millisecond?