The last challenge led to some great ideas for subtle Invisibility detection. ezzelino used a CrAu 3 spell to create a fine mist and watch for disturbances. And the practical discussion of species and penetration was invaluable.
For this week:
Archmagus Parvus, a Vim specialist of some note, has had a falling out with his Winter covenant. As a result, he has travelled to a new Tribunal, far away, to found his own covenant with "cockfights and aultrides".
Unfortunately for him, he was forced to leave behind all his sources of vis. Being not as young as he used to be, he doesn't relish the idea of spending season after season wandering about his new home, having adventures and so forth.
What spell could Parvus cast to most effectively simplify the hunt for new vis sources? Assume that he his searching for new, untapped sources rather than trying to wrest control of already controlled ones.
As always, the lower the spell magnitude the better, and try to avoid Ability checks of higher than 9+.
Where there is an aura is likely a Vis source.
InVi 2, Sense of [Magical/Faerie/Divine/Infernal] Power.
Increasing the duration to Sun makes it a 4th level spell, and so he can feel it as soon as he moves into an aura.
He's a vim specialist, so he's got Arcane Tunnel, Maintain the demanding spell, and probably MuVi abilities. "Detect vis" is a lvl 1 InVi ability. With that in mind, off the top of my head (ie, none of these seem nearly as clever as that sort of "detect invisibility" idea. )
0 - Use Hermetic Divination: "Is There a Vis Source on that Mountain" is one of the explicit examples they give. Of course, with Touch range, this is an ease factor of 10, so it's JUST outside your criteria. (specifically locating the source on the mountain is a lvl 20, though.) However, the roll for Ceremonial Divination is (Stress Die + Int + Divination + Aura + misc. Bonuses) - and you can use sympathetic bonuses via arcane connections to the target. In this case, "target" is a mountain, so it's pretty easy to get - and you can do additional things like build a scale model of the terrain, and get a map, and figure out its ancient name, and so on. So it's pretty easy to get that roll down below 9.
At the least, using just the basic "Is there a vis source on this piece of terrain y/n?" That's easy to get down to below a 9: have the grogs collect Arcane connections to various geographic features, get that map and the names of the various places, perform the ritual in the aura that you're camping in, and you're all set. This will let you determine where vis is NOT. Then, once you've identified the general locations for vis, the magi can focus his efforts there.
"Perceive the Mystic Dross" lvl 10: "Detect Vis (lvl 1), Hearing +3, Touch +1, Concentration +1. Use it on a bunch of grogs, maintain it with Maintain the Demanding Spell, and have THEM search the area for him. They'll be able to get a general area that vis is likely in, at which point the Magi can come in and identify the specific source. (Hearing is used instead of Sight because Hearing can get around things, while sight tends to be blocked.)
Consult the local genus loci (Lvl 25: Summon Airy Spirit 15, Touch+1, Conc +1) - and ask. This is probably a spell he has already.
Identify the local auras lvl 15 (identify Aura power lvl 2, Vision +4 conc+1) from the air (Levitation/flight), and then go search THOSE.
Create an air elemental and have it search for you (Lvl 30 - although this may take Vis that you don't have.) Or summon one, if you already know of its existence and name. EDIT - OK, he's an archmage: that means he's probably already created a copule that he can call on without spending Vis on. Use them.
Arcane Tunnels can be created at touch range, and maintained by concentration (and Maintain the Demanding Spell) - cast those on your grogs, as your GROGS go traipsing abound. Have one of the Detect Vis spells on each of them, and use them as sounders. So when a grog passes by a vis source, the MAGI (back in the lab) hears a beeping noise, and can figure out which grog he's hearing, and where that grog is.
Must remember to dig out my designs for the remote hermetic drone. A wonderful tool for scouting areas without sending in grogs, so long as you don't mind occasional charges of scrying if you catch another magus in it's area of effect.
(such charges are often dismissed by impartial or friendly tribunals on the grounds that it's no more knowledge than you could get by having a grog, visible, wandering the area, nor was knowledge of that magus your intention. I've personally designed them to scan for vis sources and give me an aerial overview of my local area so I know my way around better.)
Very similar to some of the approaches presented before, this is what we actually used in our last saga.
Whenever grogs went out on a mission, at least one of them carried a small circular box of mesh wire, with a bee inside. The Vim specialist of the covenant had cast a Level 5 InVi spell on the bee with R:Touch, D:Conc, T:Smell allowing the bee to smell vis (Base 1, +1Touch, +1Conc, +2Smell), and which was sustained indefinitely with a D:Ring version of Maintain the Demanding spell (any version of Level 10 or higher will do) cast around the box. Technically, he could have directly cast the InVi spell at Level 10 with D:Ring, but he knew the ReVi effect as a formula, whereas he had to cast spontaneously the InVi spell; lowering its level to 5 avoided fatigue and botch risks.
The bee was instructed to buzz and act excited, and push towards the source of the smell, when it smelled vis (think of a truffle hound). In our saga, this was accomplished when the box was created and the bee placed therein, by a grog who could communicate with bees, having the Virtue Blood of the Bee King from tL&tL. However, the same "act excited and point when you smell the vis" can be imposed (on any tiny animal, not just bees) with a ReAn spell using the Base 2 guideline "Plant a single suggestion in the mind of an Animal". This requires a just a Level 5 spell (Base 2, +1 Touch, +2 Ring, +0 Circle).
EDIT: To be fair, as players we made a mistake that went undetected for a long time: the Vim specialist made half a dozen boxes one afternoon, and then the covenant just kept using them for years - without us ever realizing that worker bees live for a few months at most. When we found out, we just fudged something about the bees' longevity being due to warping and stuff... Oh well...
To summarize what's been already said, when looking for stuff through hermetic magic, you must either:
a) have an arcane connection to it.
b) target the area in which the stuff is located - typically a large scale search requires T:Boundary, which in turn requires a Ritual (or the Virtue Hermetic Divination, as cleverly suggested by KevinSchultz).
c) give hermetic magical senses to some agent, who's then going to tromp for you around the Tribunal.
d) call up something that does not use hermetic magic (e.g. a spirit) to search for you.
a) is generally impossible for a vis hunt and b) is probably impractical/inefficient (without appropriate Virtues); d) strongly depends on the situation. This really only leaves c) as a general solution, and that's InVi Base 1, modified for Duration, Range (+1 Touch if you have long Duration or an Invisible Tunnel with all its risks, otherwise probably +4 Arc), and Target (+2 Smell or +3 Hearing are probably the best options, because they work at range and, unlike Sight, "around corners").