Receiving six days in seven, always locked but always open.
Solution: It took no little puzzling, I assure you, to understand that this riddle referenced the postbox. (No letters on Sundays, and although it is always open to deposits, it is always locked against withdrawals but when the master of the house collects the letters.)
Using the postbox key, we found a small wooden box, perhaps four inches by six, wrapped with a chain and locked fast with a 3-digit combination lock. Too we found a scrap of paper with the following description:
Guarded by spiders; seven in one, but one at a time; protect yourself lest you be harmed; under my table you shall find what you seek.
Solution: This led us — or, more properly, Eldred — to the workshop, a cobweb-infested apartment, where we found a Shop Smith table saw, and upon its steely ways, a tall wooden box.
The tall box was closed fast by no power we could discern, but closed fast it was nonetheless. At the closure of the box, opposite its hinges, three push-switches protruded from the wood, and above each switch a tiny light bulb. None of the bulbs burned, and the box appeared to respond to none of the button pushes we performed upon it, so we turned our efforts against the smaller box.
Solution: Scrutiny and argument led several of the party to the conclusion that each letter in the deranged diary entry which led us to the postbox was in fact a numeral and that some subset of these numerals might open the combination lock on the small box. Tabitha and Dash in particular were attached to this idea, but no triad of letters gave us a combination that opened the lock. In the end, I resolved that we must sum every numeral in the entire text and hope for a three-digit sum. Tabitha and Dash got to it and updated a running total at the end of each line of text. Jasper soon sensed the enormity of the eventual sum and began entering sequential values into the combination lock, starting in the neighborhood of 800. In this fashion, he got the lock open before Tabitha and Dash finished summing.
What should we find in the box but a curious device, a black circle of sorts. The diary readings led us to suppose that we had found Remy’s disgusting, unliving eye.
Solution: I can’t recall how it came about, but someone of the party surmised that the tall box was the very box of which we had read in the diary excerpts, the one containing the lantern that was William’s prison, and if that was so, then Remy’s unliving eye, bare of its eyepatch, might make it possible to work the sorcery that allowed the box to open.
Incredible though it be, gazing at the box through the unliving eye, we discovered one of the tiny bulbs — though dark as the other two to our naked eyes — glowing softly but plainly. We stabbed at the corresponding switch, and the glowing light jumped to another of the tiny bulbs. We pressed its switch next. The light continued to change places as we clicked the switch beneath its bulb, and after five clicks, the sound of a heavy lock being thrown resounded from the box. The lid lifted free!
The contents of the box was indeed a lantern of curious workmanship, and at its center, a spidery work.
The tiny bulbs on the tall box are LEDs which emit light at a wavelength of 850nm, invisible to the human eye but visible to many cameras. Remy’s unliving eye was a cell phone camera. I found that the iPhone 3 was very effective at detecting this wavelength but the iPhone 4 and iPhone 5 were completely ineffective. I started out with an old Samsung sliding phone, which worked best of all, but it broke earlier this year.
The tall box was held shut by a solenoid latch. The latch was powered by a 12V wall wart, controlled by an ATtiny84 with a TIP120 transistor. The program for the ATtiny is available on my github repo.