Cambridge University has digitized much of its collection of original Newton papers.
Newspaper and magazine articles can be especially rich primary source materials when you are researching the transmission of or popularization of an idea in the nineteenth and twentieth centuries.
When searching Primo, you can use specific Library of Congress subheadings to identify potential primary source materials:
[subject] -- Early works to 1800
Alchemy -- Early works to 1800
[subject] -- Popular works
Quantum theory -- Popular works
[subject] -- in [different domain]
[subject] and [subject]