How do I find the source materials used in "Pa. Births and Baptisms, 1708-1950"?
"This project represents Legacy extraction data available in the EASy system & data on the Vital Records Index CDs and it includes 2,605,939 records with 10,549,437 names."
You can view more detail, including images, at a Family History Center or Affiliate Library. There are birth registers, indexes. I've found some excellent information for my Philadelphia family in those digitized microfilms.