Letters The mechanism of direction See allHide authors and affiliations Science 09 Nov 1883:Vol. ns-2, Issue 40, pp. 622-623DOI: 10.1126/science.ns-2.40.622-b HORATIO R. BIGELOWFind this author on Google Scholar Find this author on PubMed Search for this author on this site Article Info & Metrics eLetters PDF This is a PDF-only article. The first page of the PDF of this article appears below.