Making and Knowing
A minimal edition of BnF Ms Fr 640 in English Translation

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Concave mirrors

A concave mirror shaped in the aforesaid form produces an endless number of illusions which seem like magic. If one wants to see oneself from behind, you have to place the concave mirror on the ground on its foot, at a slant, and then look at it one step further. But if you look at it from a closer position, it shows you the right way up but with the face quite large and the hairs of your beard as thick as

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a string and will reflect a nipple as large as a plate, and women can see the secret places they do not want to show to surgeons. It projects the reflection outwards, and if you touch with your finger the eye of the reflection, another finger will come against yours. You will be able to provide light at night for someone quite far away, if you set the concave mirror behind a candle placed on a window sill. You will be able to read and write quite far from the candle if you set it behind the candle and turn the concave face towards you. If you put the mirror at the far end of a table with a candle on it, it will reflect two lights on the table for you. To see what is happening in a street, shut the window, or the more you can shut the better, make a hole in the bottom pane, the size of your little finger, perfectly round all the way through. Then apply a sheet of paper onto the window and let the middle of the sheet be over the hole. Pierce the sheet in the same place as the hole and of the same size. Then turn the concave face of the mirror towards the hole, and anyone passing by in the street will be reflected on the sheet of paper. Do the same with the pierced door of a closed room. You can make wavy mirrors and triangular ones, such as are described by Ptolemy and Archimedes. The concave sort can also set fire to straw or light a candle from afar, using the sun. By reverberating fire too, it can provide warmth.