How can I find a very old paper by W.L. Bragg from 1913? I'm often looking for old physics papers that had a big impact on science (Nobel prize, for example). But I can't seem to find a lot of them. Is there a reason why some papers are not digitally available, but others (that had less impact) are?
Specifically, currently I'm looking for:

W.L. Bragg, The Diffraction of Short Electromagnetic Waves by a Crystal, 
  Proceedings of the Cambridge Philosophical Society, 17 (1913), 43–57.

Does anybody know where to find it online? I have access to most of the journal databases.
 A: The whole volume XVII of the Proceedings of the Cambridge Philosophical Society is available on the Internet Archive at the url

https://archive.org/details/proceedingsofcam1718191316camb

It is p.43 as marked in the text and p.60 in the document. It is available in pdf, ePub, Kindle, Daisy and DjVu formats, as well as a dedicated (very nice) online reader.
A: Here is a link to the paper - within another doc.
http://www.xtal.iqfr.csic.es/Cristalografia/archivos_10/Bragg-firstpaper-mini.pdf
A: Further to Arnold's answer, if you do find you need to go to a library, worldcat is a great resource. For this particular case, go to the Proc Camb Phil Soc page at worldcat; it doesn't look like a lot of libraries have it.
A: In general, inter-library loan is a wonderful resource for finding rare volumes and articles.
Of course, it is pretty slow.
A: Bragg's Nobel lecture ''The diffraction of X-rays by crystals'' is at
http://diamond.kist.re.kr/knowledge/nobel-physics/1915/wl-bragg-lecture.pdf
The Cambridge Philosophical Society is digitizing its old journal volumes, but this takes time, and the more recent ones are usually processed first. At present, you can find the back issues since 1920. http://journals.cambridge.org/action/displayBackIssues?jid=PSP
Thus you need to go to a library and copy it, or ask for it by interlibrary loan as in the old times. Or wait for another few months and try again.
