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That a sight-line is a straight-line, all of whose midpoints are aligned with endpoints,

That all things seen are seen in straight-lines,

That when a mirror is placed in a plane and some altitude is observed, which is at right angles to the plane, they become proportionally as the straight-line between the mirror and the observer is to the straight-line between the mirror and the altitude at right angles, so is the altitude of the observer to the altitude at right angles to the plane.,

In plane mirrors when the place is occupied at which the altitude falls from the seen object, the seen object is no longer seen.

And in convex mirrors the place is occupied though which [a line] is led from the seen object to the center of the sphere, the object-seen is seen, but the same thing also occurs in concave mirrors.
If something is tossed into a vase; and a distance is taken so that it is no longer seen, given the same distance if water is poured in, the thing tossed in will be seen.


Prop. 1: From plane mirrors and convex mirrors and concave mirrors the sight-lines are reflected in equal angles.

Prop. 19: In plane mirrors the right parts appear left and the left parts right and the image is equal to the thing seen, and the distance from the mirror is equal.

Prop. 30: From concave mirrors being positioned towards the sun a fire will ignite.