This result is due to a phenomenon of interference which occurs within the sensitive layer.
Gabriel Lippmann

The problem of direct colour photography has been facing us since the turn of the last century.
Gabriel Lippmann

In the case of composite colour, an infinity of systems must be obtained for maxima infinitely slight and with an infinity of interval values separating them - that is to say, the whole thickness of the sensitive layer is occupied in continuous manner by these maxima.
Gabriel Lippmann

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.
Gabriel Lippmann

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.
Gabriel Lippmann

During exposure, interference takes place between the incident rays and those reflected by the mirror, with the formation of interference fringes half a wavelength distant from each other.
Gabriel Lippmann

When the shot is afterwards subjected to white light, colour appears because of selective reflection.
Gabriel Lippmann