The game is called Microscope because a lot of the game involves zooming in to describe details, and zooming out to describe the bigger picture. The different levels of "zoom" are called Periods, Events, and Scenes. Events are always associated with a Period (much as the Battle of Gettysburg is associated with The American Civil War,) and Scenes are always associated with an Event (much as Pickett's Charge is associated with the Battle of Gettysburg.) You can create or destroy anything you want as part of your description just by mentioning it, even if they've never been mentioned in the game before then.
A Period is a very long (but undefined) length of time, on the scale of decades to centuries. A large war is about the smallest amount of time a Period could encompass, while something broader like a "golden thousand years of peace" would be towards the upper edge. To create a Period, you need to determine when it occurs (by placing it between two existing Periods,) and you need to describe the Period. The description of a Period shouldn't be too long; a brief paragraph summarizing what happens during this time, how it begins and ends, and how it's different from the Periods around it should suffice. Don't go into too much detail; that's what the rest of the game is for! There is always room for more time between two Periods, which is why we don't try to tie dates or times to anything.
An Event is something specific that happens during a Period, such as an important battle, a festival, a marriage, a technological discovery, so on and so forth. While Periods are broad, sweeping descriptions of everything that is going on, an Event is tied to a time and a place (though like a Period, exact times are not measured, and the literal length of an Event is not important.) To create an Event, you must place it within an existing Period, before or after any existing Events within that Period (for example, the Battle of Gettysburg Event occurs in the American Civil War Period after Abraham Lincoln is elected President, but before the Confederate surrender at Appomattox.) Then, like a Period, you must describe what specifically occurs during this Event, and how it ends. In our Civil War example, the Battle of Gettysburg Event might be described as "a pivotal battle near the Pennsylvania town of Gettysburg which turns the war in favor of the Union, ending General Lee's audacious invasion of the North." Like a Period, you shouldn't take much more than a paragraph to describe what happens; leave additional detail for Scenes. If you start writing an Event that seems to overlap an existing Event, you should probably create it as a Scene within that Event. Anything that builds up to or describes the aftermath from an Event is probably a Scene, and not a separate Event.
A Scene is the smallest unit of time, and details what happens in a specific place at a specific time with specific characters. While Periods and Events might read like chapter summaries from a book, a Scene is more like an excerpt. However, it is important to remember that Scenes should exist for a reason. As Microscope puts it, they should be "asking a question." While Periods and Events describe What happened and When, Scenes are uniquely suited to answer Who, Why, and How. The Question a Scene asks should be given as the title for that Scene; for example, the Pickett's Charge Scene within the Battle of Gettysburg might be titled "Why were the Confederates forced to retreat from Gettysburg?" with the Scene detailing the horrific casualties the Confederacy sustained as a result of the action. A given Scene should only give enough information to answer its Question; if the players want to explore Why Major Generall Pickett was ordered to Charge, then they'd have to write another Scene to answer that question. Once you know what Question you intend to answer, you place the Scene within its Event relative to any other Scenes associated with it.
When creating a Period, Event, or Scene, there are only a couple hard rules to follow:
Don't contradict established facts. As the game goes on this will get harder to manage as there will be more and more history to keep track of. If it happens that a player write something that breaks continuity, we should first try to explain it in-universe (is a character acting on inaccurate information? is there enough time between the original event and the contradiction for a plausible explanation to exist?) and only retcon a given post as a last resort.
Only write something relating to the current Focus. (More information on what exactly a Focus is can be found in the next spoiler.) You don't have to write something directly related to the Focus, but there should be some connection. To keep with the American Civil War example, if the Focus was the Battle of Gettysburg then you could write Scenes within that Event, or you could write Events that led to (or resulted from) the Battle of Gettysburg (and of course, Scenes within those Events,) or you could write a Scene about archaeologists exploring the battlefield hundreds of years later (even if the archaeologists spend more time drawing parallels to current events,) so on and so forth. Tenuous connections are fine, so long as they are clearly connections.
Don't use anything forbidden in the Palette. The Palette is basically the list of genre conventions our collective story is obeying; more on that later.
You have three days to post. If you don't post something in that time frame, we skip your turn and push you to the bottom of the list. If you miss that deadline, then you just don't get a turn this round. Please let us know if you drop out entirely so we don't waste time waiting on you.
In order to play a unified game (instead of having a dozen separate and largely unrelated stories,) all players must write something related to a specific theme which changes throughout the game. This theme is called the Focus, and it can be pretty much anything: a person, a place, an institution, a civilization, a concept like Love, Anger, or Justice, a specific Event or Period, even something you make up on the spot (though in that case you should use your turn to define what exactly you're talking about.) You can pick the same Focus again if you want to explore the same topic more thoroughly, or you can pick a related Focus to view things from a new angle. The Focus determines the direction of the game, and keeps everyone on the same page. If you're unsure what to Focus on, pick something specific and concrete like a person or a place; a narrower Focus leads to more detailed and personal history.
The Focus is declared by the current Lens. The Lens is just a fancy term for "Player One," really. Everyone takes turns being the Lens, which gives the game something of a round-based feel. Once the Focus is declared, players take turns creating Periods, Events, or Scenes related to that Focus. The Lens goes first, and has the opportunity to create two things on their turn, provided they are nested; a Period with an associated Event, or an Event with an associated Scene. This gives the Lens additional power to get the Focus going. After every player has had a turn, the Lens gets to go again, and can again create two nested items; this gives the Lens the opportunity to have the last word about their Focus.
After all players have addressed the Focus, there is a brief Legacy phase. One player (the previous Lens, aka the current bottom of the turn order) can pick an element from the history generated during the previous Focus to remember to explore later on. A Legacy is basically a thread that can tie weave throughout history from one Focus to another: a person, a place, an organization, an artifact, even a school of thought can be Legacies. There are only ever as many Legacies as there are players; if a player wants to choose a new Legacy, then they must drop their previous Legacy. This doesn't mean that the thing they chose stops existing, it just means that it's no longer "in the spotlight," so to speak.
Once the player has chosen the new or existing Legacy they want to focus on, they write an Event or Scene (but not a Period) based on that Legacy. They can write whatever they wish, since the Legacy phase has no special Focus other than the Legacy itself. Once that is written, the Legacy phase is over, and the Lens shifts to the next player. The cycle repeats.
It is in the nature of these games to not have a definite ending point; the game will last as long as there is sufficient interest in it to keep going. When you no longer wish to play, tell everyone before the start of the next Focus. That way, if it turns out this will be the last Focus of the game then everyone can use the last Focus to build to a satisfying conclusion.