The exposure glass panel of the printing frame

The exposure or insolation happens in natural light in the glass display panel of the printing frame, placed on an inclining support in order to receive the rays of light most effectively. A laboratory assistant places the negative plate in a printing frame, in contact with a sheet of sensitised paper in a silver bath.
The insolation of this frame can last ten minutes in strong light, or one day in overcast conditions…. The printing-out paper allows a permanent control of the intensity of the image, by opening one of the slides at the back of the frame in subdued light.