How can I find a very old paper by W.L. Bragg from 1913?

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.

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.

Here is a link to the paper - within another doc.

http://www.xtal.iqfr.csic.es/Cristalografia/archivos_10/Bragg-firstpaper-mini.pdf