Now we've placed two batteries under the can to make it rotate the other way. In addition to not reversing the image, it also should make the image less blurry, since now the top edge of the can has zero speed relative to the focal plane, instead of moving at twice the speed of the box. The other option was to make the slit infinitely narrow, but that would also mean pushing the box infinitely slow, and we don't have that kind of time. (Also, as we make the slit narrower, the unevenness of its edges and its slight trapezoidal shape become more significant.)