On the Rainbow: Made by the Eye, or by Sun and Cloud?
Ray geometry from the sun through the eye, and producing a bow with sprayed water in a dark room
Leonardo asks whether the rainbow ('arco celeste') is generated within the eye or by the sun acting through the cloud, and argues from mirrors that because a mirror can only receive the images of real bodies, the bow seen in a mirror must originate from the sun and cloud. He sets out its ray geometry: seen in fine rain by eyes with the sun behind and cloud in front, a straight line from the sun's centre through the eye ends at the bow's centre, so each observer sees a different bow. A ray diagram across the middle of the page shows the sun's rays, the eye and the cloud; the right margin records how blown or sprayed water in a dark place, lit by sunbeam, torch or moonlight, will make an elongated bow.
On this page
Whether the bow forms in the eye or in the sun-lit cloud
The question is whether the rainbow (its roundness) is generated by the eye or by the sun by means of the cloud. Since a mirror receives nothing but the images of visible bodies, and those images require the bodies, a bow seen in a mirror must gather the images originating from the bow itself; therefore the bow is generated by the sun and the cloud.
A straight line from sun through the eye ends at the bow's centre
The bow is seen in fine rains by eyes that have the sun behind and the cloud in front; an imagined straight line from the centre of the sun through the centre of the eye ends at the centre of the bow. No two eyes see it in the same place, so it exists throughout the whole cloud that can hold it, appearing greater or smaller, half, whole, double or triple.
Making an elongated bow with sprayed water in a dark place
If two metal spheres send the sun's rays into a dark place, water blown into them will make the rainbow ('arc of Iris') of an elongated shape. It is also made with water sprayed into a sunbeam entering a dark place with the sun at the back of the head, and likewise by the light of a torch or of the moon.