Franz Kessler (c. 1580–1650) was a portrait painter, scholar, inventor and alchemist living in the Holy Roman Empire during the 16th and 17th centuries.

Writing

He wrote a number of books and pamphlets: a book on stoves, on making sundials, on using a "sector" or "proportional instrument" (a simple calculator), See:

Painting

In the 1620s and 1630s, he was a portrait painter in Cologne, Germany.

Inventions

He wrote a book called Unterschiedliche bisshero mehrern Theils Secreta oder verborgene, geheime Kunste (Various until now mostly Secreta or hidden, secret arts), which was published in Oppenheim in 1616. The first five chapters of this book deal with communicating via a crude Aldis lamp. In the same year he built an improved diving bell. ''Unterschiedliche bishero mehrentheils Secreta oder verborgene geheime Künste. Deren die erste, genannt Ortforschung, dadurch einer dem andern die freye Luft hindurch über Wasser und von sichtbaren zu sichtlichen Orten, alle Heimlichkeiten offenbaren und in kurzer Zeit zu erkennen geben mag. Die andere Wasserharnisch, dadurch jemand etliche Stunden ohne Schaden Leibes und Lebens unter Wasser seyn kann, nach Belieben seyn Vorhaben zu verrichten. Die dritte Lufthosen, mit welchen man wunderlich über See und Wasser nach Wohlgefallen ganz künstlich gehen kann. Sammt beygefügten wohlerdachten Schwimmgürtel, welchen jedermann so wohl bey jetzt benennten Wasserkünsten, als sonst andern Wasserreisen nur Noth zu gebrauchen hätte. Alles ohne Zauberey und Schwarzkunst, den recht göttlich - natürliche Geheimnissen - Liebhabern zu gefallen beschreiben; mit Figuren geziert und erstlich an den Tag gegeben, durch Franz Keßler Conterf. von Wetzlar 1617.'' [Various hitherto mostly Secreta or hidden secret arts. (1) The first of which, called "reconnaissance", whereby one may communicate openly and quickly all secrets to another person through the open air, over water and between places visible to each other. (2) The other, a water harness [i.e., a diving bell that the diver bears on his shoulders via a harness], whereby anyone can be under water for several hours without harm to life and limb, to execute one's plans at will. (3) The third, waders with air-filled floats, with which one can go wondrously across sea and water at one's pleasure quite ingeniously. Together with an attached, well-conceived swimming belt, which anyone would have to use during the above-mentioned water arts as well as during other water-travel (in distress only). All without sorcery and black magic, written to please the truly godly lovers of natural secrets; adorned with illustrations and first brought forth by Franz Keßler, portraitist from Wetzlar in 1617.] (Oppenheim, (Germany): Hans Dietrich von Bry, 1617). A 1722 edition of this book is available on-line from the state library of Sachsen-Anhalt. Illustrations of Kessler's (1) signaling apparatus, (2) diving bell, and (3) inflatable waders.