On painting: the shifting highlight; a millstone's power
The sheen of any object is not at the centre of the lit area but moves with the eye; a note on water driving a millstone.
Under the heading 'On painting', Leonardo argues that the highlight, the sheen of any object, does not sit at the middle of the lit surface but shifts as often as the observing eye moves. A small pen sketch of an eye viewing a hatched cone illustrates the point about watching the sheen. A second note reasons by proportion that if twelve ounces of water turn a millstone thirty thousand times an hour, twenty-four ounces in the same fall would turn it sixty thousand times and grind twice as much.
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The highlight is not fixed at the centre
The 'On painting' note holds that the light of lights, the sheen of any thing, is not placed at the middle of the illuminated part. Instead it makes as many changes of position as the eye that looks upon it makes.
The sheen shifts with the observing eye
A small pen drawing shows an eye directed at a hatched cone, captioned as eyes observing the sheen. It grounds the claim that a highlight has no fixed seat but tracks the position of the viewer.
Water driving a millstone
The lower note treats a millstone turned by falling water, taking twelve ounces of water as producing thirty thousand turns an hour. It asks what a doubled flow, in the same fall, would achieve.
Double the water, double the turns
By proportion the note concludes that twenty-four ounces would drive the same stone sixty thousand times an hour, being in the same fall. It reasons that the stone would then grind as much again as with the smaller flow.