The Rosetta Stone and Ancient Egyptian
For centuries, before the Rosetta Stone was discovered, scholars tried to interpret hieroglyphs, but they could not because they thought that hieroglyphs were primitive picture writing, and that their decipherment relied on a literal translation of the image. This all changed following the Rosetta Stone discovery in 1799 CE.
While the French soldiers, who came with Napoleon Bonaparte in his campaign to Egypt, were extending a Fortress near the town of Rashid (Rosetta) in the Nile Delta, they found a stone in the old wall of the Fortress, on that stone were inscribed three texts with different languages. One of the three texts, was in Ancient Greek, and it was soon translated showing that the Rosetta Stone contained a decree by the Egyptian priests in 196 BCE thanking Ptolemy V Epiphanes,  for granting a tax exemption to the resident priests, and recorded the gifts he granted to the temples. The other two texts were the same  decree but written  in Hieroglyphics and Demotic.
Before the French could work on that Stone, they had to hand it over to the British Army after their defeat in Abukir near Alexandria, then the Stone was moved to the British Museum and has been there ever since, but a replica of the Stone remained with the French.
Attempts to decipher the text on the Stone started right away. The first attempts were by French Silvestre de Sacy in 1802. Then he handed the copy to the Swedish Akerblad who managed in 2 months to read the names mentioned in the Demotic script, but because he thought the Demotic script was an alphabetic script, he could not go any further.
In 1814, a big leap was achieved by English scientist Thomas Young, when a copy of the text fell in his hands, he realized that the Demotic script was written in signs not alphabets, then he suggested that Hieroglyphic and Demotic are related scripts. Most importantly he managed to read the cartouches of Ptolemy and Bernice.
Despite all previous efforts in deciphering the Rosetta Stone, the greatest credit is to French Jean-François Champollion (1790-1832). Champollion's interest in Egypt started very early. By the age of 12 years, he was able to read Hebrew and Arabic; later on he learned Coptic that helped him a lot when he worked on the Rosetta Stone. By the age of 18 years he worked as a Professor at Grenoble University.
In 1815, an obelisk was found in Philae with cartouches on it, that turned out later to be for Cleopatra and Ptolemy. On the base of the obelisk there were Greek inscriptions honoring Ptolemy  and Cleopatra, a copy of the text was given to Champollion in January 1822, with the suggestion of Young’s reading of Cleopatra’s name. The name of Ptolemy was already known in Hieroglyphic, and it helped to read the name of Cleopatra.  So now he managed to know equivalents of more hieroglyphic signs after being able to read the cartouches of Ptolemy and Cleopatra.
The problem was thus solved so far as the cartouches of the Greco-Roman Period, but what about those that belong to older times?  In September 1822, Champollion received copies of bas-relief of Egyptian temples with cartouches. In the first cartouche imagesCA1B0WQ.JPG he recognized the  from the cartouche of Ptolemy. Then using his Coptic knowledge he recalled the circle representing the “sun” and it is pronounced in Coptic as “re” so he has a Royal name pronounced as “re-?-ss”. That moment the name of Ramses flashed in his mind. Then he saw another cartouche with the ibis Thoth at its beginning followed by the sign  that he assumed to read ms, he immediately thought of King Thutmosis that was mentioned by Manetho. From that moment onward each day brought its new harvest. Two years later Champollion published his Marvelous work Précis du système hiéroglyphique des anciens Égyptiens solving a big part of the mystery of the ancient Egyptian language.