The principle of microscopic research (in the applied sense, of course) is based on the answer to the question: how does visible light interact with the surface of the material under study? How to use this interaction? How to register it? And how to interpret it?
Previously the need for color etching on the microstructure was conditioned by, among other things, the qualities of the equipment of that time. Sometimes it was quite difficult to recognize the features of the phase structure and determine the composition of the phases at magnifications up to 1000 times in the absence of scanning microscopy tools.
Metallography is a part of metal science that studies the effect of chemical composition and processing (thermal treatment, chemical-thermal treatment, forming, etc.) on the structure of metals and alloys.