Magnetic resonance imaging, MRI for short, uses signals that pass through fat but are absorbed by water, giving a picture as though everything but the water has been taken away. It is a picture that can tell us equally about abnormalities in the brain and the uniformity of baking of a loaf of bread. Pictures can be indirect, like the tracks of water droplets left in a cloud chamber by the scattered fragments of shattered atoms. Jet trails from a subatomic world, our main evidence for the existence of that world. Some of the fragments discovered have been put to use.
Pictures can be positive or negative.
My favourite isolated Australian beach is populated mainly by kangaroos. I have never seen them, but their tracks are in the sand, washed away each night by the incoming tide. Some tracks can be more permanent, like those of our ancestor Lucy in African sand, now set into rock. Negative images these, like the dark regions where the light has struck on a photographic negative. Indicators of former presence.