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Hi Denis
Hubble’s paper on M31 states that the measured magnitudes were photographic. Page 103 of the paper also provides some details of the comparison stars used to determine the changes in the Cepheids.
https://articles.adsabs.harvard.edu/pdf/1929CMWCI.376….1H
Hubble acknowledges additional unpublished magnitude data from Mr. Seares, who was the chief of the Department of Stellar Photometry at the Mount Wilson Observatory from 1909. Seares et al. produced the Mount Wilson catalogue of Photographic Magnitudes in Selected Areas 1-139. Allan Sandage’s paper provides a brief review of this catalogue and mentions that magnitude standards were used by Hubble for his M31 variability analysis (see Part 1. Introduction).
https://iopscience.iop.org/article/10.1086/319339/pdf
If you take a look at Part 3. A Short History, Sandage references Seares (1914, 1915) as describing the methods used to determine photographic magnitudes, while Sandage also indicates that Weaver (1946) provides a comprehensive history of the many methods used in photographic photometry in that era. The Weaver paper is available on the NASA ADS website. It is split into 6 different articles, covering various topics in more or less chronological order.
I am not an expert on Hubble’s methodology, someone else may have studied this in specific detail, but I would guess that he was using the techniques prescribed by Seares and others at the Mount Wilson Observatory at that time. From what I have seen, Hubble does acknowledge that the quality of at least some of the data was low, so yes, he would probably have had issues with poor seeing and/or mechanical issues etc. Others may know more?
Just for reference, here is a link to Hubble’s doctoral thesis
https://www.gutenberg.org/files/71654/71654-h/71654-h.htm
and to the famous Hubble M31 Var! plate
https://carnegiescience.edu/about/history/archives/plate-archives/m31var
