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On the Age of Rime, Atrus discovered crystals with remarkable qualities. Under the proper conditions, these crystals will resonate in such a way as to simulate some of the properties of Ink, allowing Atrus to construct a viewer that is capable of obtaining images from any Age, though the crystals cannot actually form a link. The correct combination of crystal shapes and colors must be determined in order to view a particular Age, and that group of crystals must be inserted in the viewer along with a blank linking Book.
Atrus made a number of improvements to his crystal viewer over the years. First, by finding a way to recreate the extremely cold temperatures of Rime (required for the crystals to resonate properly), he was able to construct a viewer that would work in the desert climate of Tomahna.
When Atrus discovered that the viewer would not function right, he went to Guildmaster Andritus on Releeshahn for help, who suggested on installing several geodes-perhaps the same kind used on rime-inside the ceiling of Atrus' observatory, and use an antenna to focus them.
Later, after discovering that interferences from the antenna redestabilized the viewer's images, (and after discovering one of Sirrus' lightning conductors on Spire) Atrus discovered that it was possible to modify the viewer to receive sounds from the Age being viewed; an ability that D'ni Books do not possess (If not to be confused with the artistic scenes of Atrus and his sons talking through the linking panels of the red, green, and blue books of Myst).
Finally, Atrus was able to add a "moving eye" attachment that allows the user to change the point of view within the selected Age (though the way it worked was never demonstrated due to an overload explosion sometime when the Stranger arrived). It is possible that this "moving eye" attachment was based upon observations of the so-called "Snowy Age" that is visible using the crystal combination in Atrus' Rime Office journal, where altering the color of one of the crystals alters the viewpoint in the Age.
The "Hearing" device works in a way similar to that of human oscilloscopes, and utilizes 3 knobs for amplitude, frequency, and phase, requiring two combined signals to match that of an age. A slider selects one of both signals, while a button shows the required wavelength.
The crystals used by the device are identified by a combination of color and shape. To operate the Crystal Viewer on Rime, simply touch the circular geode disks in front of the blank Book to activate the crystals. Touching a crystal will cause a new one to appear, while brushing your hand against it will alter its coloration. Once a combination has been set, press the large red button above the Book to activate the projector atop the Linking Panel.
The viewer in Tomahna is a bit more advanced and utilizes a second screen to select the crystal you want before generating it in the viewer itself. The right-hand control panel contains a slider and a color wheel used to select one of the crystals. There are eight crystal shapes, numbered from 1 to 8 as the slider is moved from left to right. There are six colors available: red, yellow, green, cyan, blue, and magenta.
To enter the combination needed to view a particular Age, first select a crystal using the right-hand panel. Second, press the red button beneath the compartment on the center panel that should hold that crystal. Repeat until all five compartments contain the correct crystal. Finally, press the large red button at the bottom of the center panel to activate the viewer.