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
Life is short and progress is slow.
When the shot is afterwards subjected to white light, colour appears because of selective reflection.
This result is due to a phenomenon of interference which occurs within the sensitive layer.
The series of photographic operations, developing, washing, final drying, takes about quarter of an hour.
The problem of direct colour photography has been facing us since the turn of the last century.
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.
Most of these pictures, taken while travelling, were developed on the mantelpiece of a hotel room, which proves that the method is easy enough to carry out.
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.
I got quite good results from protein plates.
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.
