From Roland Barthes’ bib⁄Camera Lucida:
For the noeme “That-has-been” was possible only on the day when a scientific circumstance (the discovery that silver halogens were sensitive to light) made it possible to recover and print directly the luminous rays emitted by a variously lighted object. The photography is literally an emanation of the referent. From a real body, which was there, proceed radiations which ultimately touch me, who am here.
The transmission of information through space is the mirroring of difference through space and time. A picture captures the difference apparent through color and location on the projective plane so that it can be re-transmitted in a different place. Hence, the medium of photography is a form of transmission of a set of differences legible to its technical mechanism of capture. Analogously, the phonograph is a means of transmission of sound waves—differences in distribution of particles in the air. The technical means of capturing some difference determines what subset of all difference is legible to be transmitted.