Despite having played quite a long time at Ars Magica, I was surprised today about the following conundrum that had never crossed my mind.
The way I had always played it, PeIm effects such as Invisibility of the Standing Wizard would make the target "invisible" as if he was perfectly transparent. But I realized this is may not be the case! PeIm destroys the target's emitted species, but not the target's ability to stop the species coming from "behind" it from an observer's point of view. Thus, a magus subject to Invisibility of the Standing Wizard would not be "invisible" in the traditional sense (i.e. "transparent"), but appear as a featureless human shaped shadow hiding whatever is in the background. Or am I mistaken?
I haven't been able to make total sense of it, either. My impression from part of it is transparency. But then you still cast a shadow, so wouldn't you be blocking light in all directions? Wouldn't that make you a human-shaped, 3-D, shadowy figure? But then you're not really invisible but just truly black (no specular nor diffuse reflection), and that's not what the spells are supposed to do. I think it is confusing. Right now I have a character using PeIm(Ig) to do true invisibility. Don't know if that's right, but it seemed like PeIm with an Ig requisite to allow for no shadow.
I think what ulf is saying is that, in the absence of magic, a wall in front of a ball does not stop the ball's species from hitting the eye, but just "overwrites" them with the wall's own species. So if you destroy the wall's species, the ball's come through as if the wall was not there.
It specifically mentions in the descriptions of the invisibility spells in ArM5 that the target still casts a shadow. Also it is mentioned in the PeIm guidelines that "...shadows are due to the physical body blocking light." and thus destroying a shadow is CrIg or ReIg.
Adding a Creo req and a magnitude seems to fix this just fine.
Or maybe I'm just not up on the nuances that have been added to the system in my absence.
Oh, btw; Hi everyone!
EDIT: Disregard the above (except for the 'Hi' part); I get it now. Ugh...
I agree with most on this thread - invisibility doesn't really work as PeIm. It should be something like Pe(Re)Im to stop the target from creating species and "teleporting" species around him. Or else, MuIm to change the way it interacts with and creates species.
That would certainly work, but it's incredibly expensive (Lev 40 base guideline!), and only works on people (you'd probably even have to include casting requisited if you want clothes to also become invisible).
I'm fine with using the spell as is. But if you just leave it there, it doesn't help you figure out how to build the spell that makes you invisible without a shadow, for instance. Meanwhile, if you understand why PeIm does what it does, you have a better chance at figuring out how to do invisibility without a shadow.