What’s meant by “stone lace of the apse of Notre Dame” taken from “1919″ by John Dos Passos?

Here’s the original context, taken from “1919″, USA trilogy by John Dos Passos:
Quote
Through the heavy lace in the windows you could see the purple branches of the trees along the quai, the jade Seine and the tall stone lace of the apse of Notre Dame.

The apse of a church is the rounded part behind the alter. In cathedrals like Notre Dame, the stone was often highly decorative – pierced by stoneworkers in lacy patterns. This person was obviously able to see Isle de la Cite and the back of Notre Dame from their window. Pax-C