I was reading a passage about 'selfie' and came across this sentence which was written on the back of the first selfie taken by Robert Cornelius in 1839. "The first light picture ever taken." I can't get the whole meaning. Actually I have a problem with the meaning of 'light' in this sentence. Any comments would be appreciated.
The two nouns 'light' and 'picture' stand in the same relationship as
The first noun describes the method of making the picture.
In this case light has been focused onto a prepared surface to make a chemical change which creates the appearance of a portrait.
National Museums of Scotland describe and display some early light pictures.