Take one small medieval cog - Target size Structure.
Secondly, make it vanish -- a PeIm effect. A house works just as well!
First question - if someone is in the hold of the ship, (or inside the house) are they invisible?
My players argue they are: however I am wondering. Here is why...
Oswald of Bonisagus, my character, has a golden crown as a talisman. It suits his must fine, but it's a bit ostentatious, so I also have a PeIm effect that can make it invisible built in. But I assume that when invisible the contents, namely my head, or the part thereof covered by the crown, remains visible...
This is intriguing. I suspect it's partly dependant on intent. My interpretation of Hermetic Magic is, while it follows scientific rules, it's not totally scientific, and is just a teeny tiny bit 'alive', or 'capricious'. After all, there are rules for a cloak of invisibility that turns you invisible also.
If you have used T: Structure -- which is the most appropriate target -- then this affects everything within the structure as well. So yes, they are invisible. Even if you had not used T: Structure, I would say yes as well (see below).
I would say that the crown itself is shedding no species, so is invisible. However, it does block light, so the hair that is covered by the crown is in shadow. No light is falling upon that hair, so no visual species are excited from it.
I think it's reasonable to assume that the contents of a vessel are invisible as well. After all, you can't see the contents of your stomach when you use PeIm on yourself. The explanation is the same as above -- visual species need light to generate and propagate them. Since light is not present, there are no species, hence invisibility of contents. So you couldn't make someone's organs visible by making their skin invisible (PeIm, T: Part) -- the organs would be invisible too. You would do this by making the skin transparant, which is not the same thing at all.
Yes, and so is he.
No, they can't see each other or the ship. They have no visual species. This is not an illusion, it is a destruction of images.
There is no easy way of which I'm aware. I maintain this as a fundamental limit of invisibility, so that it does not become too powerful*. You could use MuIm instead to make the ship look like a wave (or something); then the occupants could benefit from Discern Images of Truth and Falsehood.
*I also make invisible spellcasters cast without gestures. If no one can see the gestures -- not even the caster -- then they have no symbolic weight and thereby penalise spellcasting. The same is true of spellcasting in total darkness in my game. But this is an aside.
A possible way around the invisibility-to-themselves problem is to destroy the species at the boundary of the ship, instead of the ones generated by the ship. Presumably only the species going out, so that the ship's crew could still see what is going on outside by sensing the species entering the ship. This is a rather precise effect, so may require Finesse, or more magnitudes, or a Rego or Intellego requisite - or all of the above. It may also be impossible: Imaginem magic generally affects the generation of species IMO, not the species directly (otherwise, MuIm would have resulted in magically-altered species that would bounce off Parma).
The thing is that invisbility doesn't really make sense. Species are supposed to be stopped by objects, such as people. So the fact that I'm under a Veil of Invisibility and not shedding any species is nice and all - but I'm still blocking the view of the street behind me. It should be "Veil of Pitch Darkness", not of invisibility
The spell's description specifically says images that have been "created or altered". When using Perdo, nothing is there to be seen, so this spell can't work as described. That is not to say that there might not be a spell to detect where images should be, but that is not what Discern does.
Species are not blocked by solid objects. You can hear something that happens in the room next to you, after all. You can't normally see through solid objects because they are radiating species themselves that block your view; and species are blocked by each other. However, if intervening species are destroyed at source, the more distant ones get through.
You still cast a shadow because light is stopped by solid objects.
This might be the way to go. I had a quick scan through the fancy Imaginem effects that Jerbiton use (in HoH:S), and suspect that some sort of trick of perspective might be the way to go; however, I haven't had time to look for a full answer there. There must be some scope in redirecting the species to go outwards, or perhaps only destroy Part of an object's species. Again, HoH:S has info on what T: Part means with respect to Imaginem, I believe.
Hmm, I can hear what is happening in the room next to me, but it's muffled, and hearing something with enough stuff between us would be difficult if it's possible at all. I thought the idea was that different species advance differently through matter - sounds get muffled, colors get blocked (except for translucent materials, such as fine glass or crystal). But perhaps you're right - I suppose there is a discussion of this somewhere in the ArM supplements.
The rules do not go into specifics, specifically because these questions are 1) left intentionally open ended, b) not something that every Troupe worries about, and iii) need to be answered for themselves by the Troupes that do worry about them.
I think your premise is flawed, that you're approaching the question from an unproductive starting point - let's follow this through...
It's clear that if an "individual" is made invisible, something simply blocked by that object, behind it, is not also made invisible. So the question is - "When does 'an individual' include different things contained within it"?
If you made a wall invisible, the things inside the wall would not also be invisible, despite being "contained" (in one sense only) by that wall. However, if you made a bag of coins invisible, or a bag of wheat, that entire "individual" would indeed become invisible. The difference, for my money, is that a bag completely (or nearly so) contains the objects within, all sides.
A crown or a hat do not "contain" a head in the same way, any more than a saddle contains a rider (or the rider the horse).
Now, we move to another, less clear distinction - would a pot of gold or a bowl of porridge become completely invisible? I believe "yes", not because it's "completely enclosed" by the object, but because it's a single, discrete Individual (with regard to the Form as defined, end of page 143. And for that same reason, so would a pile of gold, or a pile of individual wheat kernels? You're not affecting an "individual" as defined by most Forms, but an "Individual-worth" of species, that much visual stuff, and so long as it's all together, collected in one space, it is one "individual" of Imagonem. That same bag or pile of gold, poured out across a floor, would not require Group, because they are not together "as one thing" - pile, bag, bowl, handful, whatever.
[i](I have problems with the "invisible cloak" that makes someone invisible - this is not, imo, consistent with Hermetic invisibility rules, depending how it's being defined. If the cloak had Range:Touch, then "one individual" can be made invisible, and that's fine. If the cloak were the target of an effect and were used as a blanket, to completely cover the person, then it's not the cloak but the entire "individual" pile of blanket/cloak/person that is actual the target and so unseen - again, fine, regardless if they're completely enclosed by the cloak or not. But if the cloak alone were enchanted (Range:Personal), then it is the only thing that disappears - the Emperor's New Clothes effect.
Non-Hermetic invisibility is the way to go for other effects that players may be more familiar with from other sources, the "visible head poking from behind an invisible cloak" effect.)[/i]
Now - if you agree that a bag of gold would work, what about a (small) barrel of gold? Sure. So why not a (small) barrel of monkeys? So long as they are in the barrel (and disregarding movement/changing image, etc), all same same. And if we move to Target:Room or Target:Structure, the same logic applies - it's all affected. And the same (on a larger scale) with a ship full of mon-, er, men.
That's where I'd start when I'd think of it, anyway.
Even on deck, I'd say that people "on" the boat are "within" the structure, since the structure includes the rigging/etc. This may be a problem to enchant since many integral parts of the "structure" you have chosen are not very durable, or you can ignore that. After all, most leather boots require resoling yearly or so, and few SG's rule that magic boots wear out. Maybe the lines are not part of the enchantment - and just hope you don't get de-masted in a storm. Ysmv.
This jumped to mind for me as well, and I bet a productive angle could be teased out of this section.
In short, if a mage is careful, by using "Target:Part" they can selectively choose which species are being destroyed. One example is an ambush - where the lurkers are all invisible in a 90 degree arc toward the road, but not sideways to each other - useful, and there are other applications that I don't remember. (And for Groups and Structures, you'd add +1 (or more) Magnitudes for "Complexity" - sorry, no, you don't get to use +1 Part to make "Part" of an entire Structure invisible, for those power-gaming sophists out there. )
The idea of destroying the species at the "border" of the structure is an interesting one. However, the ship's hull is much more clearly defined than above-decks - and when someone climbs the rigging, it becomes even less tenable. But even if you have a boat-full of crew who are visible from the waist up, that's going to be hard to spot on a horizon leagues away, and even if it is spotted it would be hard to figure out what it was. Altho' a boat with higher side-rails would screen more of the crew.
(If you're close enough to make out the details of half-a-crew, you're close enough to notice that huge boat-sized hole in the water, the boat-sized shadow next to it, and the otherwise inexplicable wake streaming from behind it!)