I am interested in looking at the notebooks, manuscripts, and letters of scientists and mathematicians from history. It is usually possible to search for a particular person, but tracking images and PDFs from many different sources can be time consuming. So, I'm looking for resource recommendations for browsing handwritten or typed informal sentiments and notes from historical scientists and mathematicians. Both online resources, and physical archives in my region (Philadelphia area), would be very useful.
For example: Michael Faraday wrote numerous letters, and his notes on chemistry have been digitized and assembled with transcriptions, into a book which is also available online. However, it is difficult to find an indexed list which allows one to search for different topics. Most scientists are much more difficult to track down than Faraday as well, e.g. Laplace, Poisson, Legendre, Riemann, Hamilton, Ampere, Maxwell, Coulomb, Hertz, etc.