A reading stone was a polished glass dome used to enlarge words before spectacles existed. Lumen is the same idea turned into a bench: a place to push light through glass and watch what it does.
Every beam on this page is traced in JavaScript, once per frame. Rays carry an origin, a direction and a wavelength. At each glass surface they are refracted with Snell's law, reflected with the law of reflection, and dimmed by a Fresnel approximation. The prism's rainbow is not painted on — it exists because blue light bends slightly more than red, and the ray tracer uses a different refractive index for every wavelength.
No images, no models, no libraries, no network. One HTML file, drawn with the Canvas 2D API and coloured by wavelength.