That's the way I'd work it too, let people write tractati up the their level and summae to half their level, as it is now.
Oh, and probably allow people to still learn from lower level tractati but do so at halved or 1/3 XP rate.
Its often valuable to see how others describe the same thing even if they´re a 20 year old apprentice and you´re a 200 year old master of the arts. But it wont give much.
Try dividing XP by the number of times you need to multiply the level with to reach the level of the reader.
So, a reader with score 21 reading a level 10 tractati gets 1/3 of the XP.
I´ve used something like that for Summae to allow them to be more useful and feel more real.
That ends up being pretty much the same as Practice in terms of learning. It seems reasonable to adjust the source quality of Practice up or down a bit depending on the presence or lack of basic facilities like laboratories, libraries of background material, scriptoria, exercise rooms, etc.. That might capture the same feeling you're going for with less complication.
Why not compare the Tractatus Quality to the Training & Teaching Gain Limits on page 165.
The Author of the Tractatus must have a higher score in the Art or Ability than the person studying it.
(this would be shudder , a House Rule)
Someone with an Ability Score of 10 can still Train a person with an Ability Score of 09 ,
as much as they can Train someone with an Ability Score of Zero.
The Maximum Teaching Source Quality for a single Student :
05+10(+02)+03+06 +05 (Apt Student) +05 (Good Teacher) = 36
(Teaching could be higher than a score of 10 of course)
- (Bonus from a Learned Magician: +03 Communication & +03 Teaching)
- (Major Essential Virtue Bonus: +06)
(The last two bonuses apply to Rolls , not Totals however)
Make studying from a Tractatus similar to studying from Raw Vis.
You need a number of them equal to the magnitude of the Score in the Art or Ability being studied.
Consider adding a Simple Die , not a Stress Die.
Even someone with 1035xp/1050xp in an Art or Ability (45-Art , 20-Ability) ,
can still gain something from Study.
Ah, that's a great houserule! It encourages much larger libraries than what we currently end up seeing, which has a nice "vibe" to it.
That is a good rule! Using the mechanics of Vis for books works well 
When you are an expert you might not learn anything from a given tractatus, since you had already figured that out yourself from other sources, for example Cool!
It totally changes how books work in ArM5, though: Quality is not important anymore, only quantity.
Xavi
That would be odd, as XP would no longer matter.
Hmm, the limit is "The teachers score in the art or ability" which is a potentially severe blow to learning Abilities.
My variant wouldnt make it a requirement that the author has a higher score, but would instead reduce the amount of XP gained. So, if you read a high quality tractatus(12) written by someone at a time when their Art score was less than 1/3 of readers current, the reader only gets a measly 3XP. Still better than exposure. And you can still learn something from reading the works of almost anyone, but if their level is too low, you might as well do something else.
I don't think I like the idea of multiple tractatuses either, mostly because it fails to make books and their authors more unique. Actually it has the opposite effect.
My proposal is that an author can write a tractatus up to the level of his ability. If the student has an equal or greater level to the tractatus, it can be used instead as a source for Practice at a Source Quality 5 (one higher than the usual for Practice). This applies also for Hermetic Arts, which can be practiced once per tractatus.
I would like to direct your attention to one cleric scholar who wrote extensively on optics during XIII century
en.wikipedia.org/wiki/Witelo
But that is the scientific method. 
Albert Einstein: "Then I would have felt sorry for the dear Lord. The theory is correct."
As quoted in Reality and Scientific Truth : Discussions with Einstein, von Laue, and Planck (1980) by Ilse Rosenthal-Schneider, p. 74
When asked by a student what he would have done if Sir Arthur Eddington's famous 1919 gravitational lensing experiment, which confirmed relativity, had instead disproved it.
(Borrowed from Wikiquote)
In any case, yes, we deliberately chose species theory because it was around in the thirteenth century and it makes Imaginem work well. On the original question, I don't see any immediate reason why you couldn't focus the sun's heat with ReIg. It might end up being higher level than CrIg to do the same thing, though.
Sorry all, I’ve been meaning to respond to this for a while now…
In retrospect it does make sense that the ancients would perceive the heat and light of the sun as separate phenomenon though this introduces another tangle in that it’s not entirely clear that heat can be manipulated with mirrors and/or lenses within the mythic paradigm. As much as I like Aristophanes’ plays, I’m not sure now credible he is on this matter considering the pleasure he seemed to take mocking the natural philosophers in “The Clouds”.
Still, let’s take a look at the proposed effect:
Focus of Helios
(Rego Ignem 25)
Range: Touch. Duration: Momentary. Target: Group.
This enchanted mirror gather and concentrates the sun's heat into a narrow beam which can be used to ignite distant objects. If the item’ user succeeds on his Aiming roll, the target becomes extremely hot and may ignite flammable objects.
(Base 4, +1 Touch, +2 Group; +10 unlimited uses/day)
By way of comparison…
Ignite the Distant Ship
(Creo Ignem 35)
Range: Sight. Duration: Momentary. Target: Structure.
This spell ignites damp wood at sight range and so presents a deadly threat to enemy ships.
(Base 5, +3 Sight, +3 Structure)
The Rego Ignem spell is 20 levels lower than its Creo counterpart. This would seem to suggest the need for additional magnitudes. Thus, the new higher-level calculation –
(Base 4, +1 Touch, +1 Concentration, +2 Group, +1 Size) = 25
The Creo Ignem spell is still higher, but has two significant advantages over the Rego Ignem version:
- No Aiming Roll is required.
- The entire ship goes up at once rather than creating a small fire that may spread naturally or be extinguished as circumstances warrant.
Suggestions or Feedback anyone?
(ReIg , page 143) For every five points by which the fire's damage exceeds +05 ,
Add one magnitude to the level of the spell.
Base Individual for Ignem is a large campfire or the hearth of a great hall , page 139 (ArM 05).
01 Cubic Pace , Base Individual of Ignem. Volume Conversion Guide (A&A , page 76).
Heat & Corrosion Table , page 181 (ArM 05).
Wood Fire has Intensity +05.
Damage is inflicted once every six seconds (once per combat round).
(CrIg) Level 05: Create a Fire doing +05 damage in an unnatural shape ,
such as in a ring or sheet or covering an item.
+01 Magnitude is +05 Damage.
As the Range is Touch , imo , you need a mirror at least one pace in diameter.
Target should probably be Part , if it is the Heat of the Sun being used and throw in a Size modifier.
Duration Momentary will give a brief burst of intense heat , but not as long as a combat round.
Damage is probably no more than +05 and applied for only a few seconds.
It may not ignite the timber on a ship , but would set the sails & rigging alight at least.
Any tar used for caulking is probably softened , but not melted.
As the magic focusing the heat is brief , i dont think a very great Distance can be covered.
Certainly not miles from a tower to ships at sea.
Cant immediately suggest a reasonable distance.
By way of non-Ignem comparison , using a craft spell.
(based on The Mystical Carpenter page 51 , Covenants)
The Mystical Shipwright , ReHe 35
R: Touch , D: Mom , T: Structure
The caster instantly makes such repairs to a ship as a skilled shipwright could make in a day.
Wood to make such repairs or canvas to replace sails and rope to replace rigging is required.
Perception + Finesse roll , Ease Factor 15 (as per page 49 in Covenants) for Fire damage.
Base 05 , +01 Touch , +03 Structure , +02 Flexibility
As the mirror isnt doing something remotely natural(it would have to be freaky huge for that(ie its not what it does that is at all unnatural, but the scale of it)), i might raise the Base level a bit. To 5 or 10.
Also, im not totally sure about the Range. Since its Rego, the Range has to be great enough to allow the spell to gather the heat used so range Sight might be a need?
On the other hand, the above 2 might be said to both be doing the same thing so applying one change might be enough.
Making the item level 35 instead. Which i think sounds fairly reasonable?
Yeah, but the item can be used by anyone any number of times, as often as you wish.
Ravenscroft and Direwolf, thanks for the comments they were very much appreciated. Insofar as many of the comments were shared, I'll try to address both of your concerns in the following response.
The mirror used is indeed substantial in size, it was part of Xavi's attempt to use the Mechanica of Heron rules to approximate Archimedes' supposed use of a giant parabolic reflector to ignite the Roman fleet besieging Syracuse. The use of the Mechanica rules effectively rules out the use of any range greater than Voice, but I don't think it should be necessary anyway as the my concept involves increasing the intensity of the heat at the mirror's surface as it is reflected - Hence the necessity for an Aiming roll.
I thought about using Part as the target, but thought Group was the more appropriate by virtue of allowing multiple... units of heat to be affected by the magic and in any case it gave the greater Target modifier.
It's the same as using Target: Part when you affect sections of the Earth ,
you don't use Group , but increase the Base Individual by adding Size modifiers.