BibTeX:
@incollection{AbdulBah1912,
  author = {'Abdu'l-Bahá},
  title = {A Celestial Mirror: 'Abdu'l-Bahá at Mrs. Thornburgh-Cropper's},
  booktitle = {'Abdu'l-Bahá in London},
  year = {1912},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.gutenberg.org/ebooks/19250},
}

APA:
'Abdu'l-Bahá. (1912). A Celestial Mirror: 'Abdu'l-Bahá at Mrs. Thornburgh-Cropper's. In *'Abdu'l-Bahá in London*. Bahá'í Publishing Trust.

MLA:
'Abdu'l-Bahá. "A Celestial Mirror: 'Abdu'l-Bahá at Mrs. Thornburgh-Cropper's." *'Abdu'l-Bahá in London*, Bahá'í Publishing Trust, 1912.

Chicago:
'Abdu'l-Bahá. "A Celestial Mirror: 'Abdu'l-Bahá at Mrs. Thornburgh-Cropper's." In *'Abdu'l-Bahá in London*, Bahá'í Publishing Trust, 1912.