Arago Rose Line
Below are two of the 135 bronze disks that form the Arago Rose Line, simply called the Rose Line in The da Vinci Code. The Arago Rose Line is better known as the Paris Meridian, the once challenger to Greenwich as the zero-longitude line.
The Arago Rose Line is named after the French astronomer Francois Arago. His recalculation of the Paris Meridian in the early 19th resulted in greater accuracy.
Finding the Arago Rose Line can be a bit difficult. I found the ones below behind the Comédie-Française near the Palais Royal. It might take time but you will find them!