Proof that the sun is hot by nature, not by virtue
The concave mirror as evidence: its focused rays resemble the sun and can burn any created thing
A densely written page arguing that the sun is hot in itself, by nature, and not merely 'by virtue' (as an incidental power). Leonardo cites the blinding splendor of the solar body, which the eye cannot fix upon, and above all the rays gathered by concave mirrors: when their focused striking is too bright to bear, it shines like the sun in its own place and no created thing withstands its heat. To the objection that a cold mirror throws hot rays, he answers that the ray comes from the sun and must resemble its cause. A top-margin note adds that such a mirror-ray passed through the windows of a metal-melting furnace, or through the sphere of fire, gains no further heat.
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The sun is hot by nature, not by virtue
That the sun is in itself hot by nature is shown by the splendor of the solar body, on which the human eye cannot fix itself. The ray comes from the sun and must resemble its cause, passing through whatever medium it may wish to pass.
Concave-mirror rays focused to a solar heat
Rays reflected from concave mirrors, when their striking is of such splendor that the eye cannot bear it, take on a splendor like the sun in its own place. If the mirror's concavity is exactly as required to generate such a ray, no created thing will withstand the heat of that reflected striking.
The ray gains no heat crossing furnace windows
A marginal note: the ray of the concave mirror, passed through the windows of the furnaces where metals are melted, does not gain heat, nor will it gain any by passing through the sphere of fire.