This is a photograph of the letter that Leonardo da Vinci sent to Sultan Beyazid II of Istanbul during the Second Florentine Period of his life. At the time, da Vinci was attempting to secure a position as a scientist and architect within the Sultan's court. Within the letter, da Vinci proposed an arched bridge from Galata to Istanbul that spanned the Golden Horn. Such a project would have been the longest bridge at the time, made possible by da Vinci's unique design. The letter was recovered in the national archives of Istanbul in 1952 and is currently in the Topkapi Museum in Istanbul.
Sources:
Atalay, B. (2013, January 22). LEONARDO’S BRIDGE: Part 2. “A Bridge for the Sultan”. Retrieved May 11, 2019, from https://blog.nationalgeographic.org/2013/01/22/leonardos-bridge-part-2-…
Books, Maps and Calligraphic documents in the Topkapi Museum. (n.d.). Retrieved May 12, 2019, from http://kilyos.ee.bilkent.edu.tr/~history/booknmaps.html
The image is taken from the source by Atalay shown above. As the photograph of a document in the public domain, it has been used with credit to the source.