Born in Toulouse in 1750, died in Paris in 1819 Pierre-Henri De Valenciennes was a landscape painter or paysagiste and professor at the Beaux-Arts school. An expert on perspective, he writes a widely influential manual on Chromatic Perspective:

Éléments de perspective pratique à l’usage des artistes: Suivis de réflexions et conseils à un élève sur la peinture et particulièrement sur le genre du paysage, Paris, l’auteur; Desenne; Duprat, 1799

Elements de Perspective online (In French)

Chromatic Perspective, or Aerial Perspective or Atmospheric Perspective, is the technique of creating an illusion of depth (perspective) by depicting distant objects as paler, less detailed, with softer edges. It is used almost exclusively in landscape painting as it aims to represent natural optical and atmospheric conditions.

A precursor was Leonardo Da Vinci, who paints, in Mona Lisa for example, objects bluer the further back they are in the painting. The method was extended for all colors by mathematician Girard Desargues (1591-1661) and his disciple, artist Abraham Bosse.

Pierre-Henri De Valenciennes‘ paintings are often difficult to date, but his works of the 1790’s onward often look like they were painted a century later.