The length of exposure (one minute in sunlight) is still too long for the portrait. It was fifteen minutes when I first began my work. Progress may continue.

In a paper delivered to the Philosophical Society of Glasgow on 17 January 1883 John G. Extremely fine-grained high-resolution photographic emulsions are inherently much less light-sensitive than ordinary emulsions so long exposure times were required. His father Isaïe a French Jew born in Ennery near Metz managed the family glove-making business at the former convent in Bonnevoie.