A law reveal the inevitable:

where the two objects contact will create a transfer phenomenon. Will take away some of the things, also left some things. - The father of forensic Rocca who will be left behind traces through. - The father of forensic pilocarpine

一条定律，道破必然

凡两个物体接触，会产生转移现象。即会带走一些东西，亦会留下一些东西。

——法证之父罗卡

凡走过必留下痕迹。

——法证之父艾德蒙·罗卡The Locard exchange principle, also known as Locard's theory, was postulated by 20th century forensic scientist Edmond Locard.

Locard was the director of the very first crime laboratory in existence, located in Lyon, France. Locard's exchange principle states that "with contact between two items, there will be an exchange" (Thornton, 1997).

Essentially Locard's principle is applied to crime scenes in which the perpetrator(s) of a crime comes into contact with the scene, so he will both bring something into the scene and leave with something from the scene. Every contact leaves a trace.

“ Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value. ”

—Professor Edmond Locard

1910 年，法国警官艾德蒙罗卡建立了一套黄金定律，那就是人类无论做过何种接触，一定会留下微迹证。他的报告奠定了现代刑事鉴识科学的基石

利用罗卡的原理，刑事鉴识人员分析犯罪现场所发现的纤维，就可查出大量线索，如衣服款式、衣服售出地、甚至连织品制造商都可以查出来。藉由分析微小的纤维，微迹证鉴识员能够确切指出嫌犯曾身处现场，进而定罪嫌犯。

《法证先锋2》的第14集曾提到过。