by Leonardo da Vinci
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Every body in light and shade is situated between 2 pyramids one dark and the other luminous, one is visible the other is not. But this only happens when the light enters by a window. Supposing a b to be the window and r the body in light and shade, the light to the right hand z will pass the object to the left and go on to p; the light to the left at k will pass to the right of the object at i and go on to m and the two lines will intersect at c and form a pyramid. Then again a b falls on the shaded body at i g and forms a pyramid f i g. f will be dark because the light a b can never fall there; i g c will be illuminated because the light falls upon it.
Light and shadow with regard to the position of the eye (141?145).
141.
Every shaded body that is larger than the pupil and that interposes between the luminous body and the eye will be seen dark.
When the eye is placed between the luminous body and the objects illuminated by it, these objects will be seen without any shadow.
[Footnote: The diagram which in the original stands above line 1 is given on Plate II, No 2. Then, after a blank space of about eight lines, the diagram Plate II No 3 is placed in the original. There is no explanation of it beyond the one line written under it.]
142.
Why the 2 lights one on each side of a body having two pyramidal sides of an obtuse apex leave it devoid of shadow.
[Footnote: The sketch illustrating this is on Plate XLI No 1.]
143.
A body in shadow situated between the light and the eye can never display its illuminated portion unless the eye can see the whole of the primary light.
[Footnote: A stands for corpo (body), B for lume (light).]
144.
The eye which looks (at a spot) half way between the shadow and the light which surrounds the body in shadow will see that the deepest shadows on that body will meet the eye at equal angles, that is at the same angle as that of sight.
[Footnote: In both these diagrams A stands for lume (light) B for ombra (shadow).]
145.
If the sun is in the East and you look towards the West you will see every thing in full light and totally without shadow because you see them from the same side as the sun: and if you look towards the South or North you will see all objects in light and shade, because you see both the side towards the sun and the side away from it; and if you look towards the coming of the sun all objects will show you their shaded side, because on that side the sun cannot fall upon them.
The law of the incidence of light.
146.
The edges of a window which are illuminated by 2 lights of equal degrees of brightness will not reflect light of equal brightness into the chamber within.
If b is a candle and a c our hemisphere both will illuminate the edges of the window m n, but light b will only illuminate f g and the hemisphere a will light all of d e.
147.
That part of a body which receives the luminous rays at equal angles will be in a higher light than any other part of it.
And the part which the luminous rays strike between less equal angles will be less strongly illuminated.
Gradations of strength in the shadows (148. 149).
148.
That part of the object which is marked m is in the highest light because it faces the window a d by the line a f; n is in the second grade because the light b d strikes it by the line b e; o is in the third grade, as the light falls on it from c d by the line c h; p is the lowest light but one as c d falls on it by the line d v; q is the deepest shadow for no light falls on it from any part of the window.
In proportion as c d goes into a d so will n r s be darker than m, and all the rest is space without shadow.
