1.http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Snell.html

2.http://ws.rice.edu/ES/humsoc/Galileo/Catalog/Files/snell.html

3.http://physics.hallym.ac.kr/reference/physicist/Si.html

4.www.britannica.com

5. Physical Science; Glencoe/McGraw-Hill;pg. 764; McLaughlin and Thompson; Copyright 1997