Observers might ask the logical question, "How does a fake-medieval fantasy guy see microorganisms anyway?"
The answer: Baron van Hook's Microcosmic Eye.
As part of his plant biology studies, Baron Lee van Hook determined that he needed to be able to see how plants work in miniature. How do these things fit together? What action pulls food and water from the soil?
When a magnifying glass wouldn't work, he turned to specialized applications of magic to answer his questions. It took a lot of research, but Baron Hook finally developed specialized scrying methods to view the microcosmic world around us.
Persistent rumor suggests Baron Hook is researching a refined version of the spell, Baron van Hook's Fundamental Eye. He suspects there are things even smaller than what he can see with his microcosmic eye. Modern observers might infer that the fundamental eye would have resolving capabilities similar to electron microscopy.
Baron Hook would likely be willing to teach the spell to others (tell your FLAILSNAILS friends to come to the Sorrowfell Plains!), but no one knows because most people consider his microcosmic investigations to be somewhere bordering on quackery. It is likely that he has already taught the spell to some of his colleagues or that it has been otherwise archived at the Wizard's Tower.
What follows are three versions of the spell, one for retroclones, one for D&D 3e, and one for D&D 4e.
Magic-User Level 1
Duration: 1 turn/level
This spell augments the caster's sight, allowing him or her to magnify objects. While focusing on an object, the caster can "zoom in" to a magnification as high as one thousand times, essentially the upper limit of modern light microscopy. As an added benefit, the spell automatically enhances contrast, replicating some of the utility of microbial staining.
Among the typical components required of magic-user spells, this spell requires a small, finely-crafted lens worth 100 gp.
Third Edition Version:
Level: Brd 1, Sor/Wiz 1 (the jury's still out as to whether any clerics have channeled this spell, but the Knowledge domain seems like a good bet)
Components: V, S, M, F
Casting Time: 1 standard action
Duration: 10 min./level
You can magnify objects by sight and will alone. While focusing on an object, you can "zoom in" to a magnification as high as one thousand times, essentially the upper limit of modern light microscopy. As an added benefit, the spell automatically enhances contrast, replicating some of the utility of microbial staining.
Baron van Hook's microcosmic eye can be made permanent with permanency. The caster must have a minimum caster level of 9th level and spend 500 XP.
Arcane Material Component
A pinch of crumbled, dead leaf litter.
A small, finely-crafted lens worth 100 gp.
Fourth Edition Version (Ritual):
As you finish the ritual, you find that a microcosmic world opens for you.
Time: 10 minutes
Duration: 1 hour
Component Cost: 5 gp, plus a focus worth 20 gp
Market Price: 50 gp
Key Skill: Arcana
For the ritual's duration, you can "zoom in" your sight at will to see microscopic objects and creatures. At the DM's option, this may allow you to re-roll a failed Perception check — as well as a failed Nature or other knowledge check — when appropriate.
Focus: A finely-crafted magnifying glass you look through as part of the ritual.