Postulates of perspective and the diminishing of distance
A ray's straight path, a proportion of colours, degrees of diminution, and a distant sunlit mountain
Leonardo opens with a postulate he asks to have granted in the proofs of his perspective: that every ray passing through air of uniform rarity travels in a straight line from its source to the object it strikes. He then poses a proportion of colours (if one ounce of black with one of white lead yields one degree of darkness, how many degrees will two ounces of black to one of white make?) and works out the degrees of diminution on a perspective screen set one braccio from the eye, where an object four braccia off loses three-fourths of its height, eight braccia off seven-eighths, and sixteen braccia off fifteen-sixteenths. Further notes observe that a small near thing and a large far one look equal at equal angles, ask why distant things still appear large though the screen shows them small, and consider how well the eye sees a non-luminous mountain according to where the sun stands behind it.
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A ray travels in a straight line
Among the things Leonardo asks to be conceded in his proofs is that every ray passing through air of equal rarity goes by a straight line from its cause to the object, or point of impact.
Proportion of colours
If one ounce of black mixed with one ounce of white lead makes one degree of darkness, how many degrees of darkness will two ounces of black to one ounce of white lead make? The problem is set out as a proportion of pigments.
Degrees of diminution on the perspective screen
With the perspective screen one braccio from the eye, a thing four braccia away loses three-fourths of its height on the screen, one eight braccia away seven-eighths, and one sixteen braccia away fifteen-sixteenths; when the distance passed is doubled, the diminution is doubled.
Equal sizes at equal angles
A small thing nearby and a large thing far off appear the same size when they are seen at equal angles. A companion note asks why far things still seem large though the screen proves them small.
Seeing a distant, non-luminous mountain
Leonardo asks how well the eye can see a distant, non-luminous object such as a mountain: it is seen well enough if the sun is on the far side of it, and appears nearer or farther according to where the sun stands in the sky.