In the number system, your references are listed on a separate references page in the order in which they were cited by first appearance in your text, and they are numbered accordingly. Each source is, of course, simply listed once, even though it may have been referred to in the text numerous times. The type should be double-spaced, lines should not be skipped between each reference, and a hanging indent of five spaces should be used after the first line of each reference. Always include the word "References," boldfaced, in the center at the top of the page.
Sample Number System References Page
1. M. Poulain and J. Lucas, "Optical Properties of Zirconium Tetrafluoride-Based Glasses," Mater. Res. Bull. 10, 243 (1975). G. E. Rindone, "Influence of Platinum Nucleation on the Crystallization of Lithium Silicate Glasses," J. Am. Ceram. Soc., 41  41-42
2. Gutzow and S. Toschev, "The Kinetics of Nucleation and the Formation of Glass Ceramics," in Advances in Nucleation and Crystallization of Glasses. Edited by L.L. Hench and S.W. Frieman. American Ceramic Society, Columbus, OH, (1971). pp. 10-23.
3. C. Walker, "Kinetics of Nucleation and Crystal Growth in Glass Forming Melts in Microgravity.<http://searchpdf.adobe.com/proxies/0/34/70/56.html>. accessed May 10, 2001.