During my last session of game a player asked my Storyteller if a weapon extracted from its scabbard BEFORE the cast of the Invisibility spell becomes invisibile or not, or if a weapon inside a scabbard count as a single discrete thing (target: Individual) and therefore subject to the spell's effects or not. Otherwise does he need a casting Requisite (Terram)?
Thanks for your answers!
That is a positively excellent question.
First, you're not going to make a sword invisible by casting a spell on its scabbard if the sword is not inside it when the spell is cast - no question about it.
A person and its equipment (within reason) are a single Individual with respect to many spells, so I'm comfortable with a sword and its scabbard likewise forming a single target, at least with respect to Imaginem spells - which make no distinction of substance.
There are no need for requisites, invisibility is always pure Perdo Imaginem regardless of what you're making disappear: it's always just species that you're destroying.
Now, what happens when the two are parted? Should a dropped invisible sword become visible again? I'd say no. The target was important when the spell was cast, but not to keep it in existence. You could cut an invisible rope in two and both halves would remain invisible.
I'm fuzzier on the question of how to treat the contents of an invisible box. A sword and its scabbard are quite straightforward in comparison, in my opinion.
Basicly this spell destroys the species of a single individual (usually a person), anything that leaves the individual under the effect will no longer have any new species destroyed, and thus become visible once more.
So, an item carries at the time of the casting og the PeIm spell reverts from invisible state once dropped...Does it become invisible if picked up again? I think not. Same goes for completely new items picked up while invisible, the should not suddenly become affected by the spell. But how do species travel? If I'm invisible, and pick up an apple, completely affected by the magic, what then? I just said it shouldn't become invisible, so the apple'll send out species. But if i hide it under my cloak, will the species find their way out anyway? Are species completely insubstantial, as to pass through an invisible barrier, while the same barrier in visible, non-tampered with state will block them, sending out its own species?
Hmmmmm.
I'm not so sure about the dropping/picking up effect on invisibility, but I think I can answer the 'hiding under cloak question.' (serf's parma):
Since an invisible character still casts a shadow I'd say that his body still blocks species. Therefore I would rule that an apple hidden under your invisible cloak cannot be seen. (unless your have special 'see through cloak' abilities 
)
This does bring me to the strange point in invisibility: what do others see?
Since you still block species from behind you, somebody would see a black sillhouette of the 'invisible' Magus. This of course nullifies the whole invisibility concept, so how does this spell fit into magic theory?
hm, some second thought stuff on this one:
Perhaps the sun's rays are Ignem instead of Imaginem, and that is the reason they are still blocked. If you do not affect species, you are not turned black, but still cast a shadow. But then the apple also shows.
Aaaargh, now I'm confused. 
Light and darkness are ignem, yup. So the sun is ignem stuff. Good catch about a reasoning of why you cast a shadow when youdo not block species from other sources.
About the apple, I would say that the apple emits species normally, but that they are blocked by the cloak and your body, if you hide it under them. The species of the apple are emited normally, but they stop when they hit the cloak as usual. .... Dunno if this last part makes a lot of sense....
Xavi