We present ultrahigh precision measurements of the optical heterogeneity of high quality fused silica at the δn=10-8 index level. The measurements were made with a unique scanning heterodyne interferometer developed as part of the Gravity Probe-B Program. Striae of refractive index variation Δn<10-7 (optical path difference smaller than 5 nm) are clearly visible on an extended heterogeneity background of around Δn=6×10-7. Both striae and extended heterogeneities are repeatable to within a few parts in 108. These results show the nature and extent of the heterogeneities of high quality vitreous silica.