The Hemlock Gallery
The Stained Glass at Sainte Chapelle
The Isle de la Cite in central Paris has two superb examples of high Gothic architecture, the Cathedral of Notre Dame and the much smaller Sainte Chapelle.  What makes the latter outstanding is its stained glass windows, which account for most of the upper part of the structure.  The impact on a bright day is literally dazzling, and we can only wonder what the effect would be on a the devout of the ill-lit and probably drab Middle Ages.

The chapel was built in the 1240s by Louis IX to house supposed relics of the passion, notably fragments of the crown of thorns. He paid much more for the relics than he did for the chapel. The chapel was used by the royal family, whose ownership of such relics was supposed to represent a divine seal of approval not extended to lesser nobles. The royal link led to vandalism and neglect in revolutionary and early Republican times, and it was only in the mid-19th century that badly needed repair work and restoration took place.

Despite restoration, much of the actual glasswork is original. The windows themselves show hundreds of scenes from biblical stories. 
The Catholoc Encyclopedia on Gothic Architecture
Links on Sainte Chapelle and other Gothic Architecture
A French
architectural site
Among the many things the Internet cannot do: justice to the magnificence of stained glass windows
Basic tourist information
From the outside.  The chapel is enveloped by the Palais de Justice
All window and no wall.  These windows are some 45ft tall and 13 ft across.
Rose window. 
The interior woodwork is almost as colourful as the windows