Primary source: pubmed.ncbi.nlm.nih.gov/13357554/
Verified locator: Primary abstract: H. E. Karrer, Journal of Biophysical and Biochemical Cytology 2(4 Suppl), 287–292, 25 July 1956; DOI 10.1083/jcb.2.4.287; PMCID PMC2229681. Three mouse-lung electron micrographs show variable apparent width between alveolar epithelium and endothelium; oblique sectioning is explicitly a possible explanation for some thickening. PubMed abstract inspected; full scanned paper not inspected.