I initially thought you will only need to design a specific teleportation spell (teleporting you to the target you will be touching - weird, I know), but after reading the description of IT, I am not sure it is even possible.
IT allows you to cast spell with a "range greater than Personal on that target". So even if you designed the spell I mention earlier, it won't work because:
the TP spell is still a Personal range spell
IT does not give "Touch" in the physical sense, it only makes the target sensible to "Touch" range spell, no matter where she is. And the target of a TP spell is the person being teleported, not the landing spot.
Maybe if you design a TP spell with a Vim requisit it could become possible: "Leap through the Void" (ReCo 35 (for unlimited TP distance) +1 for vim requisit =ReCo40, pers, Inst). Teleport the magus toward the target connected through an IT.
If the TP spell counts like being cast through the IT, then the IT need to be of matching level but it can be debatable since you are not casting a spell at the target of the IT, but merely using the conduct as an AC... I still believe having an hairlock and the standard TP spell would be more efficient - especially if it is grogs.
Also, note that Seven League Stride (ArM5, page 135) lets you teleport to a place within range that you can see. So, as long as you were within seven leagues (21 miles) of it you should be able to teleport to the other end of the tunnel (using Seven League Stride), once you have cast an effect down the tunnel to see the other end.
This is another of the fairly long-standing discussion topics in ArM5: if and when you can see through magic something you could not otherwise see (e.g. because it's in a dungeon on the other side of Mythic Europe), can you affect it with R:Sight spells?
I'm sure it's been debated a lot. I don't know if any official ruling on this has snuck into the errata or any source book recently. Hey, if not maybe can we ask David Chart for errata/clarification on this too?
I think it is much different than voice. Voice says that magically enhancing the voice does not increase the range. sight says that anything a mage can see which would include magically increasing your sight.
Intangible tunnel does not grant sight through it. It says that in the spell. " The tunnel does not, of itself, grant any sense impressions of the target"(pg 162) While you can cast magic through the tunnel to the target you can not sense the target in any way unless you cast another spell that would allow you to do so.
Which is pretty strange when you think about it. How do you actually target a Target that you can't sense? Usually this would seem to violate the Limit of Arcane Connections, although since the base version of the Tunnel works at AC range, perhaps that covers it. The Sight range version from Hermetic Projects sounds even more problematic, unless the Target remains in sight for the entire duration of the Tunnel and not just at the time of casting.
Being at Touch range but being unable to sense what you're touching is a little weird.
It sounds like it. Essentially then Intangible Tunnel is just a metamagic "cheat" enhancing an existing Arcane Connection, such that Touch or greater range effect are possible. I don't think the Sight range version should work, since there's no AC to enhance.
I tend to agree with you, except if you can have a metamagic cheat for AC range, why not other ranges? The AC allows one to target an unsensed target, but if I'm seeing someone off in the distance, I should be able to cast a sight range spell. If that spell is Intangible Tunnel, why can't I then use the tunnel to cast a spell on someone I can still see, but as if he were at a lower range? I'm not a big fan of the who cast at Sight range and then have them move out of sight range and still be able to cast spells concept...
I suppose there's no fundamental reason why not, I'm just musing that the nature of Intangible Tunnels is to affect Arcane Connections specifically.
If you did allow a Tunnel without an AC, I do think the sensory limitations should come into play. If the Target moves out of Sight range, magical senses should be required to target it with additional magic, even if the Tunnel exists for a long duration.
In those words all AC targets are a cheat. Also remember that Intangible Tunnel works both ways. Your target can cast spells at you like an AC but using the link you made.
It's a "cheat" in the sense of bending the usual game rules, as all the Vim metamagic is. It's not a cheat in a bad way. Just weird, like most Vim.
You have to sense the target (or have an Arcane Connection for an Arcane Connection Range version) to cast any of the Intangible Tunnel type spells in the first place.
Once the Tunnel is open, you don't need to sense the target of the Tunnel, because that's what the Tunnel does. You need to sense your local Tunnel terminus, instead. The caster of the Tunnel automatically senses his local terminus.
There's some appeal to that argument but I disagree. It makes sense if you're just casting down the Tunnel as some sort of pipe, but that's not actually the way it works. You're affecting the Target at touch range, possibly including by effects requiring precise manipulation (Finesse) or targeting only Part of the Target. I don't think this works through just sensing a Tunnel terminus, and the spell is explicit that no sense impressions are passed by the Tunnel spell. So the magic must be allowed by Arcane Connection, so as not to violate the Limit.
The implication is that the magus better be careful not to break his Arcane Connection if he wants to keep using a Tunnel.