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While you discuss your next stop, Apple Basket takes a moment to exchange some farming expertise with Chipper Sprout. After a few minutes, you agree to follow the stallion’s suggestion, and visit the factory.
 
You bid him farewell, then exit the greenhall, and merge into a small crowd of ponies whose shift just ended. Soon, you make a turn toward a building seemingly as massive as the one you came from, and enter what appears to be a similar loading bay.
 
Inside, you find a few empty carts, and scattered groups of workers here and there. When you stop to ask them for directions, they guide you to the lightworks department. With their help — and that of multiple map screens —, you manage to navigate the building’s countless corridors until you reach the door to your destination.
 
Behind it, you find a pink mare working on some sort of large cylindrical gemstone. She greets you with a confused, yet amiable look.
 
Mare: “Hello there! Are you lost?”
 
You introduce yourselves to the mare, and tell her that you were told to come here to learn more about the Undercity’s ceiling windows.
 
Mare: “Really? What a specific thing to be interested in! Well, you’ve come to the right pony; my name is Cinnabar Spark, and I’m the one in charge of making them.”
 
Cinnabar Spark offers you a quick tour around her workshop; a veritable maze of conveyor belts, crates, furnaces, machines, and tables, with just enough space to walk in between.
 
Cinnabar Spark: “So! The big windows you’ve been seeing are called oranoscopes. They are a recent addition to the Undercity, actually, and they were originally designed by Bright Logic, who currently runs the library.
 
In essence, they’re large glass panels linked together by long tubes of silfrite mirrors, with an input that extends all the way up to the surface.”
 
The mare grabs a piece of pipe from a nearby counter. Though the exterior is round, four flawless mirrors are seated on the inside, creating a reflective square-shaped interior.
 
Cinnabar Spark: “This is what the tubes are made of. Now, you may or may not have noticed, but every oranoscope shows the same image of the sky. That’s because they’re all branching out from the same outside source.”
 
Cinnabar Spark then leads you to a table on the other side of the room. On that table, you find some sort of circular stand, and a piece of grid paper with colours forming no discernible pattern. Hanging from the ceiling above is what appears to be a lamp.
 
Cinnabar Spark: “Naturally, the images we get down here are fragmented and distorted from hitting all those mirrors. For example, this grid here is an inverted colour map of how white light is coming out from an installation in progress.”
 
The mare then retrieves the gemstone she was working on earlier, and places it atop the stand.
 
Cinnabar Spark: “This is a correction prism. As the name suggests, this intricate cylinder is designed to correct the image’s output using refraction.”
 
Smoke: “Looks delicious.”
 
Cinnabar Spark: “It’s actually a container with hundreds of smaller solid glass prisms inside. So unless glass is part of your diet, I wouldn’t try eating one!”
 
Seeing that the dragon isn’t going for it, Cinnabar Spark grins, and turns on the lamp above. A beam of light shines through the prism, and illuminates the paper below with a three-dimensional rainbow, causing most of the colours on the paper to disappear, and others to change.
 
Cinnabar Spark: “This white light projector allows me to preview the prism’s output. When the colour map appears all white, that’s when I know that the prism is done. But as you can see here, this one still needs some tweaking.”
 
The mare rotates the prism to demonstrate, making colours shift all over the grid. Your party seems entertained by the colourful display, even if you don’t all understand quite how it works.
 
Cinnabar Spark: “Finally, when all that’s done, a magnifying glass is mounted over the whole thing to match the size of the input panel on the surface. I don’t have one of those to show right now, unfortunately.
 
And that about does it for oranoscopes! They’re not perfect, of course, but they bring us sunlight all the same. Pretty simple over all, huh?”
 
Moonflower: “That’s so cool! If bat ponies had these, it would make livin’ in caves so much easier!”
 
You thank Cinnabar Spark for her time.
 
Cinnabar Spark: “You’re welcome! And if you’re interested in making your own oranoscope, feel free to copy some blueprints from the library.”
 
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