ARTICLE PDF ENJOYING OUR LATEST CONTENT? LOGIN OR CREATE AN ACCOUNT TO CONTINUE * Get immediate access to this article * Also access the latest journalism from Nature's award winning

team Access through your institution or Sign in or create an account Continue with Google Continue with ORCiD RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS

ARTICLE Dr H. W. Parker. _Nature_ 220, 1155 (1968). https://doi.org/10.1038/2201155b0 Download citation * Issue Date: 14 December 1968 * DOI: https://doi.org/10.1038/2201155b0 SHARE THIS

ARTICLE Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard