Ophthalmology and philately: IV, Ophthalmologic Congresses.--The 21st International Congress, 1970.
D003226:Congresses as Topic / Q000266:history*; D005060:Europe; D049673:History, 20th Century; D006801:Humans; D008800:Mexico; D009885:Ophthalmology / Q000266:history*; D010678:Philately; D014481:United States