You probably know about the telescope in Riven, or Gehn's Fifth Age. It is standing right next to the linking chamber. My guess is that it was put there for Gehn to observe the condition of the Star Fissure. However, I guess Gehn did not know that a careless person could break the glass sealing the Fissure if he lowered this telescope too much.

*SPOILERS*
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Gehn built the telescope right after he found himself trapped on Riven. He thought that he could escape Riven through the Star Fissure, so he built the telescope to experiment and observe what happened to the people (alleged Rivanese transgressors) he would throw into the Fissure (what a nice guy, uh?). Those thrown into the Fissure (apparently) didn't died, but Gehn never managed to figure out what really happened to them.
Gehn knew that the Fissure's seal could only be broken deliberately (by removing the mechanical stop that prevented the telescope from hitting the glass window), and that doing so would most likely mean Riven's collapse, so i guess he figured no one would ever want that and that he didn't have to bother with a better safety mechanism for the telescope beyond the 'stopper' and the code. Obviosuly, he was wrong.
I'm glad the telescope ended up where it did after 'falling' through the Star Fissure when Riven collapsed ;)

More SPOILERS! Don't read unless you've played all of the games.
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The first person to fall through the fissure was Atrus before the events of Myst (seen in the intro). Luckily he had his Myst book and used that to escape... but he didn't know what would become of the book.

The book eventually ends up somewhere and the player finds it. This is why in all the endings of Riven you fall into the fissure... it leads to the player's home. We know this from the fact they were able to find the book in the first place.

As Namur sort of mentions, the telescope ends up in the desert you're in at the start of URU. This makes me think this is also where the book was found.

Fun fact(ish): IIRC the first four Myst games are set in the 1700s (according to the totally canon novels co-written by the Miller brothers).