Recent digitization of records continues to reveal more information about Muybridge.
In 1851 or ’52, Edward James Muggeridge emigrated to America, initially settling in New York. In the autumn of 1855, he moved to San Francisco, and about this time became Muygridge.
On 7 November 1856, Muygridge applied for the first stage of naturalization as an American citizen. His name is recorded in the Index to Declarations of Intention for Citizenship 1851-1906 [ACR 2534535]. [Accessed through Ancestry.com] (The original Index page has his name in manuscript, transcribed as ‘E.I. Muygridge’. The original page number in the Index is: 147. The index entry notes: ‘Bk: 2 page:21’ – presumably the volume and page number of the original court application record.)
In order to become a naturalized citizen at that time, it was necessary to have resided in the United States for a minimum of five years. The first stage of the application (registering intention) could be made after two years, and three years later the final application could be made.
In 1856 Muybridge (to use his more familiar name) had been in the country for four or five years, and had evidently decided to stay when he made the initial application. He was still in the United States three years later (late 1859), at which time he could have applied for the final stage of naturalization. Whether he did or not isn’t yet known, and discovering the answer will require research in the US. (Any volunteers?)
In 1860, Muybridge returned to England. Some time in the Spring (or later) of 1866 he went back to San Francisco.
If Muybridge failed to make the final application for naturalization in 1859, or when he returned after 1866, it may be that he did so before he married Flora (in 1871), as an American woman who married an alien apparently lost her U.S. citizenship, even if she never left the United States. Or at least, her citizenship was held in abeyance (she adopted the nationality of her husband) during her marriage.