I suspect an invisibility rethink is in order.
MuCo
Transparency of the Body, Lvl 15
R: Touch, D: Conc., T: Ind.
Turns a person as transparent as air, rendering them invisible. A discerning eye may observe a slight outline, similar to the boundary of water and pure alcohol.
The spell affects only a human body. For clothing, requisites are required.
Base 5+1 (Conc.) +1 Touch
That base is very loose guess. I wonder if an Imagonem requisite is needed?
Such an effect may be possible, but I think the Base is way off.
My guess is that it would be much closer to Base 30 "Turn a human into an insubstantial object (Auram requisite)", since you are giving the target a property of the air.
Intuitively, invisibility through MuCo should be quite a bit harder than through PeIm.
Otherwise, you could still go the Perdo way with this. PeCo 40 "Destroy one property of a person, such as their weight or solidity", the property in this case being being visible.
There is an existing spell that does invisibility somewhat like this for objects: A Window of Singular Direction (MoH:TL p.141).
What about using the level 3 Rego Ignem Guideline (ArM5 p.143 box), applied to light instead of fire, hence "Move light quickly through space while behaving naturally"?
Couldn't this be used to pass light through a target from all sides to all sides - hence making the target transparent to light like air.
That would then be a ReIg Base 3, +1 Touch, +2 Sun (and perhaps +1 affecting a moving target) spell: level 10 (or 15).
This would just destroy the visual image of the target - with perhaps some trickery at the boundaries, where the airlike transparency meets real air.
Affecting the other senses requires still PeIm.
Sounds more like Base 10 "Control a fire in a very unnatural fashion". Light does not naturally goes around something.
Plus, it would leave the target blind, as no light would reach him.
Edit: Might even leave a dark spot visible.
Could indeed be ruled that way, especially if the light is actually moved through the target, as I wrote above.
Not, if the light is moved through it, right?
Moving the light through should avoid the dark spot, too.
But allowing light to move through something isn't simply moving the light -- it's changing the properties of the thing it's moving through, isn't it?
Unless you are in fact changin the properties of the light so that it can move through something, in which case that's more in the realm of MuIg. Not an easy thing either.
How? The properties of the target, as far as they block the light, should just be overridden by the magic moving the light (perhaps in an unnatural fashion), shouldn't they? The moved light would likely register as magic, though.
You mean teleporting the light, instead of letting it pass through?
I rather meant passing it through - so that the light "beams" keep their directions and still reach the eyes of the target, if such exist. This really might be covered by at least Base 10 "Control light in a very unnatural fashion", mightn't it?
To be clear: this is not something every table should adapt. But once we start revising invisibility, it is a logical proposal. And very luckily, there is tmk no real understanding of the working of the eye in Mythic Europe yet - so light affecting it and still passing through it should be possible, at least in an unnatural fashion.
Just to be clear: what is the Target (big T) of the spell you are proposing? It seems it should be a light source illuminating whatever-should-be-made-invisibile. Or rather every such light source, which would require Group or Room, depending on the circumstances.
It would have to be the light surrounding the target to be made invisible, independent of its sources. ArM5 p.143 Light Shaft of the Night also does not target its light sources: stars and the moon.
How about MuIm?
A body of air, MuIm Lv 4
R:Touch, D: Son, T:ind
Makes a person and all his accoutrements look as if he were made of air, rendering them invisible - though a careful observer may notice a shimmer if the target moves.
Base 1 +1 Touch +2 Sun
Clearly too easy for game balance, but shouldn't it be legal?
Hmm. That's Group though (and it's a bit "tricky", an adaption of a legacy spell).
I don't know, it seems ... very difficult to say the least.
You have to transport the light just right, from all directions at once.
Yep. That requires a definition sounding solid, like: simply "passing it through".
No added features: Finesse requirements get deadly here.
Not sure what you are referring to. That's MuCo. I am talking about MuIm. Way easier as long as you only modify appearance (including the lack of Requisites).
It still has the shadow issue showing up. Just because you change the person's image doesn't mean their body doesn't block light.
The reference was explicitly to MuCo, because MuIm doesn't affect light.
Note that it has been errataed and is now level 15.