1. Abstracting Periodicals: According to Allent Kent “an abstract is a summary of a publication or articles accompanied by an adequate bibliographical description to enable the publication or article to be traced.
H. M. Weisman defines “the abstract is an announcement medium whose objectives is to provide knowledge of and an ordered and logical access to new primary literature”.
An abstracting periodical is “a regularly issued compilation of concise summaries of
i) Significant articles (often in a very limited subject field) that appear in current primary source journal and
ii) of important new research monographs, reports, patent and other primary source publication in that field”.
Example: Library and Information Science Abstract, London, Library Association, 1950-, Bimonthly. Indian Science Abstract, Delhi, Insdoc, Vol. 1-, 1965-.
Besides the above sources, list of periodicals, list of theses, dissertations, location and finding list etc also serves as bibliographical sources.