Sometimes, like the moments leading up to when a police officer decides to shoot someone, transparency is an unalloyed good. And especially lately, technology has progressed to a point that it makes this kind of transparency not just possible, but routine. So it is in Rialto, Calif., where an entire police force is wearing so-called body-mounted cameras, no bigger than pagers, that record everything that transpires between officers and citizens. In the first year after the cameras' introduction, the use of force by officers declined 60%, and citizen complaints against police fell 88%. It isn't known how many police departments...