The standard way of using Imaginem in the Ars Magica is to cast a spell that manipulates the species an object emits (either by creating an image that emits species, as is standard for Creo, destroying them for Perdo etc). Is it possible, though, to affect the species striking an object, rather than emitted by it? And if so, how would targets work - could you affect the species striking a human with an individual target?
The only example that's spring to mind is that it's possible to ward an object against species. That wasn't what I was originally after, though (which was synethesia via Imaginem), and in any case wards have a tendency to work slightly differently to a lot of Hermetic magic.
You could always ward someone against species. But making someone blind or deaf is so powerful it's hard to allow it. I never found a satisfying answer.
There's also some selective invisibility spells in HoH:S p64, but they require Finesse.
I did consider the Transparencies in HoH:S examples when thinking about the issue to begin with, but they don't do what i want either - they're still destroying the species emitted by a target - albeit only some of them, as dependant on where another target is, rather than affecting the species at the point of hitting the person you're hiding from.
On the ward against species balance issue - actually, is it possible? I'd been blithely assuming there was a guideline for it, but I can't see one even in the Columbae section of HoH:S. Destroying someone's minor senses is a base 15 guideline, a major sense 20, although Perdo Corpus won't need a duration longer than momentary.
According to Magic Theory, it should be possible, even if the guideline doesn't exist. Maybe a base of 10 for sight and 5 for other senses, once you add a decent duration it should be balanced with PeCo.
For that matter, you can ward the target against light (which carries visual species) or air (which carries auditory species).
As for manipulating the species just before they reach someone's eye or ear, I think the greatest difficulty there is that you normally do not sense those species yourself (you may sense other species emitted by the same object, but they are not the same). You can create a miniature illusion to override them (a louder noise, a stronger smell). I am not sure it would work with an image though, the original species might bleed through.
Probably not any more than with "normal" illusions.
And if you fear this, the Cr(Pe)Im is the way: destroy the old species, create the new ones.
Or MuIm (maybe ReIm, serf's parma): change the old species into the new ones.
The original idea I had was that I wanted to use Muto on all the iconic (i.e. sight) species hitting a target, and turn them into echoic (sound) species, thereby introducing a nasty case of synethesia on the victim. Essentially an attack/personal defence spell for a Muto Imaginem specialist which was a bit different from the more standard "I change what X looks like, thereby misleading you".
Then I realised that I couldn't actually think of a good way to do it. I think Halancar's right; sensing the species is going to be a major problem. According to page 63 of HoH: Societas, magi may manipulate either the capacity of an object to emit species, or the species directly, but I suspect the latter is only practical for Creo, due to the sensing issue. And possibly Intellego, but I'm not sure I can do anything with Intellego that'll enable me to use the other Techniques.
[As a side note, creating an illusion only one person can see is possible - see Miniatures on pages 67-68 of HoH:S. Essentially, you create an illusion with Part target and an extra magnitude for flexibility which only sends out species in one very specific direction, which you control using Finesse. The species from the illusion therefore are only emitted towards the person you want to see the illusion. As well as being a good tool for convincing people they're going mad, it can also be used for things like sending private messages. However, they're usually resisted by magic resistance, as you tend to be creating the species directly rather than an image that emits natural species.]
Editted to add: The problem isn't only sensing the species, but the fact that you'd have to affect species that don't exist yet, and you therefore cannot possibly sense.
One interpretation that fits all Techniques is that Imaginem only affects the "emission" of species. Creo creates a source of species, Intellego senses the source, Muto changes what the source emits, Perdo stops the source from emitting and Rego move the source elsewhere. That also explains why the species aren't affected by MR.
I am not happy with that interpretation as I too would wish to affect a sensor rather than an emitter, but I haven't found anything that fits right.
you are defining a boundary with an ongoing effect that affects species crossing it - sounds like a ward to me.
If you have the penetration then Muto Mentem is probably easier for synthesia and if you don't then you can't attach your zone of synthesia to the target and they can just step out of it. Unless...
What if you stuck the effect to the outside of the enemy's parma? Cr(Mu)Vi(Im) A perfectly elastic, flexible, immaterial membrane that turns light to sound on inward passage. Conjure it with zero penetration as wide bubble around the enemy. Being perfectly elastic it contracts and if it meets no resistance it contracts to nothing and only magic resistance stops it so it shrink wraps anything with MR. It exerts no inward pressure nor resistance to movement but nor does it tear or abrade unless it is the target for a suitable PeVi.
