(since critical hit damage is precision damage).No, it isn't.
Each attack of a scorching ray can crit. It just can't all get sneak attack (or sudden strike, or skirmish) damage.
Right I went through and reviewed, Critical damage is not listed as precision damage... my understanding of what the complete arcane said was:
You can crit with a damage dealing spell (we know this)
You can sneak attack with a damage dealing spell (we know this too)
Spells with multiple hits only get extra damage to the first hit.
I took that critical hit damage is extra damage and therefore only applies to the first hit, just like sneak attack damage only applies to the first hit.
I'm thinking that the Errata for complete arcane covered this too, but I'm too lazy to go through and look it up for sure.
That and it followed in line with the unstated policy of, "Casters aren't allowed to do decent damage with spells."