Circa 1720–1730. Red Chalk Drawing. Portrait Of Christian Wolff, Mathematician And Philosopher

Christian Wolff (1679–1754) was a key figure in 18th-century thought, a mathematician, physicist, and philosopher who drew the wrath of Frederick William I for his atheism but was honored by Frederick II, who restored his professorship and appointed him vice chancellor of the University of Halle.
This eminent thinker constantly sought to synthesize the entirety of knowledge through the lens of mathematics. 
He brilliantly popularized Leibniz’s theories.
He remains a central figure of the Enlightenment on the other side of the Rhine.
This drawing appears to have been engraved (the Paul Getty Foundation owns a copy).
I attribute it to J. Ruber (or Raber) because we are presenting, further up, a similar drawing that was engraved in France by Étienne Ficquet, depicting Jean Bernoulli, the famous mathematician. 
Unfortunately—and curiously—it has so far been impossible to find any trace of this artist. Perhaps this is due to an erroneous transcription of the name by that famous engraver.
The drawing is of very fine quality, executed in red chalk. 
It is set in a very simple frame made around 1920, on a mat that appears to be original.
Dimensions of the drawing: 13 cm x 8.5 cm