The plate at each point only sends back to the eye the simple colour imprinted. The other colours are destroyed by interference. The eye thus perceives at each point the constituent colour of the image.

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.