> If it's bright, the rods and cones send no signal, if it's dark, they send a strong signal. It's inverted. There's apparently a very good reason for this but I don't remember what it is.

The reason is to prevent light fatigue in eyes. Ears and nose experience a quick fatigue when exposed to the same stimulant for a long time. With inverted arrangement in eyes, you have a naturally stimulated inhibition rather than a fatigue inhibition.