Duhem The Aim and Structure of Physical Theory pp 145-147
Go into this laboratory; draw near this table crowded with so much apparatus: an electric battery, copper wires wrapped in silk, vessels filled with mercury, coils, a small iron bar carrying a mirror. An observer plunges the metallic stem of a rod, mounted with rubber, into small holes; the iron oscillates and, by means of the mirror tied to it, sends a beam of light over to a celluloid ruler, and the observer follows the movement of the light beam on it. There, no doubt, you have an experiment; by means of the vibration of this spot of light, the physicist minutely observes the oscillations of the piece of iron.
Ask him now what he is doing. Is he going to answer “I am studying the oscillations of the piece of iron carrying this mirror?” No, he will tell you that he is measuring the electrical resistance of a coil. If you are astonished and ask him what meaning these words have, and what relation they have to the phenomena he has perceived and which you have at the same time perceived, he will reply that your question would require some very long explanations, and he will recommend that you take a course in electricity.
It is indeed the case that the experiment that you have seen done, like any experiment in physics, involves two parts. In the first place it consists in the observation of certain facts… In the second place, it consists in the interpretation of those facts…
The result of the operations in which an experimental physicist is engaged is by no means the perception of a group of concrete facts; it is the formation of a judgement interrelating certain abstract and symbolic ideas which theories alone correlate with the facts really observed.