The chapter`s title is another instance of a ZUN wordplay. It's a reference to "Hotaru no hikari, mado no yuki." (glow of fireflies, snow on the window) which is the Japanese version of Auld Lang Syne. The expression "snow on the window" means to study under adversities, and has its origins in the Chinese poet Chen Tzu-Lung, who was said to have to read his books at night in winter by the moonlight reflected on the window's snow, because his family was too poor to buy lanterns or candles. And of course, both a phantasmal light and a lesson hard-learned are references to what happens in the story.