I have the nagging feeling this has already come up in a thread, but I can't find it... so I'll start a new thread about it (new voices can also chime in this way). Let's consider a few scenarios.
a) A hut is divided in two rooms by a central wall. A magus is in one room. Can he see into the other room by making the central wall invisible?
b) A magus is wearing a pouch full of silver coins. He makes the pouch, but not its contents, invisible. Do observers see the coins?
c) A magus is covered head to toe by a hooded cloak. He makes the cloak invisible. What can observers see?
In Ars Magica, invisibility by Perdo Imaginem destroys the species emitted by an object. However, it does not make the object transparent: the object still blocks light and thus creates a shadow (this is straight from the corebook) and since light is what carries species (I think this is from A&A) it stands to reason that an invisible object still prevents whatever lies behind it from being seen. Thus, an object made "invisible" by Perdo Imaginem should be literally invisible, but easy to detect as a dark featureless "hole" in the sensorium of the observer. The effect would be akin to a text with censored words: one can't read the censored words, but one can easily and precisely sense their position and size.
This interpretation has the advantage of being consistent with Ars Magica "physics", and very easy to adjudicate (in scenario a), the magus sees just a dark, utterly featureless wall; in scenarios b) and c) observers get to see dark, utterly featureless silhouettes of a pouch and of a cloaked figure, respectively). It has the fatal disadvantage that what you get by no means corresponds to the intuitive "invisibility" of ancient and modern tales. Can you help me recover "classic" invisibility while maintaining consistency with Ars Magica "physics"?
PeIm Invisibility spells do not just delete iconic species, but also pass along others in their stead. The invisible object still blocks light and casts a shadow, though.
Potentially, it can still completely prevent light from reaching other objects in that shadow, which hence still cannot be seen. That doesn't make the invisible object itself a "dark featureless "hole" in the sensorium of the observer", but requires constant adjustment of the spell to the movement of the invisible object, and hence adds complexity to the spell: see ArM5 p.146 Veil of Invisibility. Invisible beings also better be careful, not to cast too complete a shadow onto something.
EDIT: For ArM5 invisibility to make sense, one needs to strictly separate light (Ignem) and iconic species (Imaginem). An object needs to be exposed to light to generate iconic species. An invisible object still blocks light, but does not generate own iconic species any more, and passes through the iconic species generated by other objects,
B & C ) the contents (coins or the people) would not be seen as the species of the contenant are destroyed (of course, shadows etc still exists), yet as the object doesn't generate species (or they are destroyed, either way) the contents still generates species. i.e. the human under the cloak generates species, those are physically blocked by the cloak so you don't see the person but the cloak.(yet the form lets you guess there is a person, but that's not the point) so if you turn the cloak invisible, you block the species of the cloak making indirectly the person invisible (of course if he shows his hands or face out of the cloak they wouldnot be affected are the species of the person are not block by the spell but by physics.
A ) this one is tricky, but it depends on how you turn the central wall invisible. If you modify the species so that they are not blocked physically by the wall, the walls is effectively invisible and you can see the other side (but won't be able to move there). but if you destroy the species coming from the wall on your side, in my interpretation you won't see the other side, although you won't see the wall too (it would be interpreted as a very dark place by the mind)
If either room is dark, no. Otherwise, I think yes. As mentioned above, the invisible wall does not allow light to pass through but does allow species to pass through.
I don't think so, because it is dark inside the pouch. But if the magus deliberately does not have the spell affect the coins, the coins still block species, so the pouch will look weird, and if the pouch is large enough, something is obviously wrong.
A magus-shaped shadow.
Just cast the PeIm on the magus, which includes his cloak! Cast PeIm on the pouch without deliberately excluding its contents. The magus casts a shadow, but is invisible. The pouch disappears, but mythic observation might notice the impression it makes on the magus' robe, how his belt is slightly pulled down by its weight, how it is every so slightly darker in one little spot.
Hmm, this does not sound entirely right to me. As far as I know, historically two related but different theories of (visual or "iconic") species were formulated:
Light is needed for objects to generate species; species then travel on their own to the observer's eye.
Objects continuously generate species, which are carried by light to the observer's eye.
It seems to me that you are assuming the first theory holds in the ArM5 universe, whereas HoH:S (p.61) seems to support the second ("Species are particles that are continuously emitted from objects ... Iconic species are carried in light...").
This means then that an object that blocks light from passing through should also block visual species from passing through, regardless of whether it's "PeIm" invisible or not. This is the essence of the problem.
So in the first scenario (wall separating two rooms) even if the wall is made invisible it still blocks species from the other side from passing through, because it blocks light from the other side from passing through (to make this clearer, think what happens if the magus' room is in complete darkness, but the other room has a bright fire burning in it).
No, an invisible object does not block iconic species. See for that ArM5 p.146 box Perdo Imaginem Guidelines:
You are over-interpreting a single line on HoH:S p.61 in a way, which goes against ArM5 core rules and makes invisibility by Hermetic magic unworkable.
So lets go through your three problems:
If the room beyond the central wall has light in it even with the wall in place: yes. Though shadows may look strange there.
If that room has no light in it with the wall in place: no. No light is getting into it still.
Assuming, that the coins do not get any light within the pouch: No.
Assuming, that the magus does not get any light while wrapped in his cloak, that hence the magus' face and eyes are completely covered, and that no light falls onto the magus' feet: Not the magus.
A really tricky issue for invisible magi in plain sight is the area underneath their feet, though: it will be reached by very little light, and stay very dark with the magus on top invisible. Experienced magi will notice this.
It doesn't per se. But it does block light, and iconic species do not travel unless carried by light.
I don't think I'm "overinterpreting" anything. HoH:S is very clear on the subject. And it's not just "a single line" as you instead say; the notion keeps getting repeated (e.g. later on the same page "[species] are also limitlessly available, because all objects emit a continuing stream of species.") So, your view that (iconic) species are only emitted when an object is struck by light, but do not need light to be carried to the observer, is repeatedly contradicted by canon sources. It's fine as a house rule, and it solves a lot of other problems, of course! But the original post was asking if invisibility by Hermetic magic could be preserved without violating anything in the current Ars Magica line.
The more I think is that the source of all troubles with this issue is the ability of invisible objects to block light. Why was it ever introduced? After all if something blocks light, it is affecting the sight of an observer, apparently contradicting the PeIm invisibility guidelines.
Remove the ability of an invisible object to block light, so that a PeIm'ed object is really transparent, nothing seems to break and all contradictions appear resolved. I think that if it's impossible to work out a solution reconciling all Ars Magica sources on the subject without contradicting any of them, this would be my house rule of choice.
AFAICS, HoH:S p.61 "Iconic species are carried in light .." is the only few words in HoH:S p.61ff Species, that can be - with a lot of good will, a modernist idea of light rays, and little sense for ArM5 physics - interpreted as "an object blocking light also blocks iconic species, no matter what kind of magic goes on around". Note there especially p.63 "Magi, using their Arts, may manipulate either the capacity of an object to make species, or the species directly." So they clearly can also make an invisible object pass through other species, so as to (ArM5 p.146 box) "Destroy an object's ability to affect sight".
"Iconic species are carried in light .." is certainly not meant as you read it, but just describes the medium light as necessary for iconic species to exist. Otherwise ArM5 p.146, especially the box, would have been errataed in many places.
ArM isn't about authors trying to hash out alternative physics and its interworking with magic to utter precision and everybody's comprehension, but still describes a game which requires a little common sense to work.
No. See the "Species Magic Example" on p.63 too, for just another example of the notion that iconic species (that are constantly emitted regardless of light) are carried by light. Plus, there's logic: since in several places we read that species are constantly emitted, regardless of the presence of light, if light were not the necessary medium for them to travel in, why would we not see in darkness? Frankly, the text seems pretty clear on this issue. And it's not modernist thinking, it's an ancient (and in fact rather incorrect) theory of light and vision.
No. Let me state it again. HoH:S repeatedly states that iconic species can exist regardless of light. They need light to travel.
Now, as I keep saying, does this contradict other material? Probably, and that's why I posed the original question: "can you help me resolve the apparent contradiction?" You seem to be so assured that it creates a contradiction, that you are just claiming that the text in HoH:S must be wrong (or, more precisely, you are claiming that HoH:S says something utterly different from what it quite clearly says). This is not very helpful: as I already said, if I need to houserule the issue away I definitely know how to do it.
To get over some first issue, which you repeat, though it looks utterly irrelevant for this thread to me:
Whether an item in the dark generates iconic species, that immediately cease to exist or to travel because of the absence of light, or this item does not generate iconic species any more, is an issue for a conventicle of scholastics or Imaginem specialists.
The same also holds true for the way to determine the identity of species.
This makes wording about moving species around a bit easier for me - so lets stick with it.
The relevant tidbit to make ArM5 Perdo Imaginem and HoH:S Species work together is, that Imaginem magic can move iconic species through invisible objects and put them on their way through light again. This does neither contradict ArM5 p.146, nor HoH:S p.61ff Species. It actually gives a reason for ArM5 p.146 box "Destroying changing images is more difficult — add one level of magnitude to spells that do so."
So some Perdo Imaginem magic does not just destroy species.
Depending on what "see in the dark magic" really is, there are several ways.
ArM5 p.131 Eyes of the Cat works only in "near darkness".
With ArM5 p.113f Magical Senses you can create spells - e. g. ArM5 p.153 The Miners Keen Eye - peering into utterly dark spots with T:Vision. These magical senses need to penetrate MR, and create their own iconic species.
By ArM5 p.114 "a magical sense that works with vision is no good in the dark." So even iconic species created by magic require light for your eyes to receive them.
There may be other ways. What do you envision?
EDIT: HP p.84 InIm 5 Fingers for Eyes is a clever way to magically perceive a room without Magical Senses, and potentially even if you are blind.
IIRC, there isn't a see in the (complete/total) dark spell. Eyes of the cat is low light, or near darkness. Eyes of that bat allows you to feel the boundaries of air...
Synaesthesia spells convert one kind of species into another species, which, if they convert into iconic species, may not work in total darkness, unless these spells are designed with ignem requisites to provide light...
Perdo Imaginem magic must allow visual species to pass through objects for invisibility spells to work as intended. The species inside the invisible object are not illuminated while inside the object, but once the species pass through the invisible object, ambient light will illuminate those species once more and they become visible.
However, since invisible objects block light, making a container invisible is insufficient to allow one to see the contents, which is not illuminated. If a container is truly opaque, all you will see is a black mass; a coin pouch or cloak might not be truly opaque to light and so some detail might be perceived.
If you want to see the contents of an object through a container, use Intellego (Form) magic or add a Creo Ignem requisite to your Perdo Imaginem spell.
Personally, I rename Veil of Invisibility -> The Living Shadow, and introduce a new invisibility spell based on ReIm to delicately "teleport" the species hitting your body to the right positions so that they'll effectively go "through" you. (With a Perdo requisite to block your own species.) This also has the advantage of making invisibility more difficult to pull off.
Veil of Invisibility ReIm(Pe) 35 (base 4, +2 change image along with object, +1 complexity, +1 requisite, +1 Touch, +2 Sun)
But it's a house rule. The above is an excellent way to treat the issue without house ruling it.
I do think some of these things bring up some big questions about species. Let's see what we have:
Strong Faerie Blood: You can see in darkness or semi-darkness.
Second Sight: You can see invisible things.
Eyes of the Cat: You can seen in near darkness but not absolute darkness.
InIm base 3: You can see in darkness.
See in Darkness (MoH): (I don't have access to my book.)
First, there is a lack of clarity: semi-darkness vs near darkness vs darkness vs absolute darkness. The first two may be the same as each other, as may the last two, but maybe not. If light is needed to transmit visual species, how can anyone see in absolute darkness. But if darkness is not absolute darkness, then we're OK on this one (unless the text of See in Darkness is explicit about absolute darkness).
Second, Second Sight is the real issue. It doesn't need to penetrate, so it should just be the possessor using their eyes relatively normally. But let's say we're using PeIm to destroy our magus's ability to give off visual species. What species are being picked up by the person with Second Sight?
Third, spirits can be naturally invisible but could potentially be seen (such as via Second Sight). Does this mean there is essentially an invisible color or group of colors? So when something is invisible it is giving off visual species that are just not normally picked up by eyes? This handles part of the issue with Second Sight but still disagrees with the PeIm wording.
So at the moment I'm pretty sure no one interpretation will actually fully work. It seems like somewhere something has to fall apart.
If the sight in SS is metaphorical, it is no problem. SS also lets you 'hear' things too. Perhaps it circumvents species; someone with SS magically perceives certain things. InCo(Im?) cast on a room might similarly show what invisible people look like. Maybe. But yeah, any ability that lets a person 'see invisible' in AM has to deal with the lack of species to see.