Sunlight through an aperture; mirror and reflected rays
Optical ray diagram with a compass method for measuring a distance
On blue paper, a large pyramid of converging rays dominates the sheet, studying how a beam of solar light passing through a narrow chink is dimmer than one lit by the full diameter of the sun, in proportion to the width of the opening. Leonardo labels a true mirror and its lines for the sun and for the reflection, and gives a compass-and-straightedge construction, dropping a perpendicular to a base line and striking arcs, that leads into a method to measure a distance by drawing parallels and a convergent line. Small marginal sketches of an instrument and vessels sit at the upper left.
On this page
A beam lit through a narrow chink
a b is brighter than c d. The point t, lit through the narrow chink from the side of the sun o, is as much less lit than if lit by the whole diameter a b as o is smaller than that diameter a b.
Lines of a mirror for sun and reflection
The true mirror is, for the sun, 3 and 4; for the mirror itself, 2 and 6; and for the reflection, 5, 1 and a. The lettered rays sort the incident sunlight from the reflected beam.
Compass construction to measure a distance
Draw the perpendicular from the point of convergence a to the base line 40 and 50 at b, then set the compass on a and b and describe the arc b 3, narrowing the compass to the span b 2. To measure a distance, set the convergence at a and draw the parallel lines c b, S o and t p, then draw the convergent line a o.