Charles Cros

In 1860 Cros began studies in medicine, but he soon abandoned them for a life of literary and scientific pursuits. In 1869 he published a theory of color photography in which he proposed that a single scene could be photographed through glass filters colored green, violet, and orange. The three negatives obtained through those filters could be developed to produce positive impressions that contained varying amounts of red, yellow, and blue (the "antichromatic" or complementary colors of the filters). The three positive impressions, when superimposed on one another (for instance, by making three carbon prints using sufficiently transparent pigments, then transferring the pigmented gelatin onto a single support sheet) would recompose the original colors of the photographed scene. Cros's proposals, which anticipated the subtractive method of modern photography, were similar to more influential ideas advanced about the same time by Louis Ducos du Hauron. The same day, May 7, 1869, Charles Cros and Louis Ducos du Hauron presented their method of creating color photographs to the French Society of Photography. They had not been in communication beforehand and each knew nothing about the other's research. Cros ended up conceding the invention to Ducos Du Hauron, despite having deposited a sealed paper at the French Academy of Sciences on December 2, 1867. Ducos du Hauron had patented his ideas on November 28, 1868, almost a full year later, but claimed to have written an unpublished paper on the subject in 1862.

Cros is perhaps most famous as the man who almost, but not quite, invented the phonograph. As far as is known, no one before him had thought of a practical way to reproduce sound from a recording of airborne sound waves. He gave the Greek name 'Paleophone' ('voix du passé', tr. 'voice of the past') to his invention. On April 30, 1877 he submitted a sealed envelope containing a letter to the Academy of Sciences in Paris explaining his proposed method. The letter stated in French, "Un index léger est solidaire du centre de figure d'une membrane vibrante ; il se termine par une pointe [...] qui repose sur une surface noircie à la flamme." The English translation is one close to this: "A lightweight armature is fixed to the center of the face of a vibrating membrane; it ends with a sharp point [...] which rests on a lamp-blacked surface." This surface is integral with a disc driven by a double movement of rotation and linear progression. The system is reversible: when the tip follows the furrow the membrane restores the original acoustic signal. The letter was read in public on December 3 following. In his letter, after having shown that his method consisted of detecting an oscillation of a membrane and using the tracing to reproduce the oscillation with respect to its duration and intensity, Cros added that a cylindrical form for the receiving apparatus seemed to him to be the most practical, as it allowed for the graphic inscription of the vibrations by means of a very fine-threaded screw. An article on the Paleophone was published in "la semaine du Clergé" on October 10, 1877, written by l'Abbé Leblanc. Cros proposed metal for both engraving tool attached to the diaphragm and receiving material for durability.

Before Cros had a chance to follow up on this idea or attempt to construct a working model, Thomas Alva Edison introduced his first working phonograph in the US. Edison used a cylinder covered in tinfoil for his first phonograph, patenting this method for reproducing sound on January 15, 1878. Edison and Cros apparently did not know of each other's work in advance.

Cros was a member of the group known as the hydropathes which existed around the period 1878–1881. Charles Cros, played by Christopher Chaplin, appears in the film Total Eclipse, about the lives of Paul Verlaine and Arthur Rimbaud. Cros is seen for a few seconds at the Le Chat Noir in Paris, a café which opened in 1881 and had become the home for the avant-garde art scene of the time.