Canons: light and force

Introduction

This week covers the eight optics entries and the mechanics entries in Canons II and its Explanation. It is the most astonishing part of the book: people in the fourth century BCE recording, in the fewest possible words, the behavior of shadows, pinholes, mirrors and the lever.

The eight on light. "A shadow does not move": the light changes; the old shadow dies and a new one is born. "Two shadows": two sources. "The image is inverted; at the crossing there is a point." "A shadow facing the sun": by reflection. "The size of a shadow depends on the slant and distance of the source." Then three mirrors: plane, where images "are many yet seem few"; concave, upright and enlarged inside the focus, inverted and reduced outside; convex, always reduced and upright.

The Explanation's account of the pinhole is the finest passage in the book. "Light reaches a person like a shot arrow," traveling straight. "The lower part of the person becomes high, the higher part low": the feet block the light from below and so form an image above; the head blocks the light from above and forms an image below. On 16 August 2016 China launched the world's first quantum communication satellite and named it Micius, 墨子号, for this passage.

Mechanics. "Force is that by which a body is set moving." On the balance: "add weight to one side and it must tilt." When counterweight and load balance, "the root is short and the arm long"; add equal weights to both and the long side must drop. This is the law of the lever, before or alongside Archimedes.

墨门 / The Mohist Community does not read this to argue about who discovered what first. We read it to see one thing: the test of "what people see and hear," followed seriously, becomes experiment.

The three tests

The optics and mechanics rest entirely on the second test: watching how a shadow moves and how an image flips. But "like an arrow," light traveling straight, is not seen; it is inferred from the inverted image. Only straight-line travel puts the feet above and the head below. That is the Canons' "inference" at work: observation supplies the phenomenon, reasoning supplies the mechanism. This week's question sits right there: a scientific conclusion usually needs both witness and inference, and the three tests name only the first.

For families

• 科技堂: make a pinhole box from a shoebox, foil and tracing paper and look at a window through it. Why is the image upside down? Have the children draw the rays.
• 科技堂: make a lever from a ruler and an eraser and find where the pivot must be for one coin to lift two. Read "root short, arm long."
• Tell the story of the Micius satellite and ask why a passage more than two thousand years old would give a satellite its name.

For chapters

• 20 min reading · 20 min 墨辩 · 10 min 三表 review · 10 min 职 reports.
• Replace this week's 墨辩 with a "demonstration debate": two people give a ten-minute demonstration with a homemade pinhole box and lever, the others question them, and the group sees whether the text of Explanations II alone can reproduce the results.
• 三表 review: using the pinhole, separate which step is witness and which is inference, and discuss how the 科技堂 should teach children to tell the two apart.