Tuesday, May 17, 2011

I LLLove Rainy Days

Bringing an umbrella all the time is not a waste.
Sun cream is no longer necessary.
The street is cleared, and gains the rare quietness.
Most important, rain washes the world.

So let it pour,
To wet the earthworms and salamanders in the spring;
To disperse the sear and desiccated air in the summer;
To fill the fruits with richer juice in the autumn.
To  indicate a warmer day in the  winter.

Monday, May 16, 2011

Locard's Exchange Principle

This principle has repeatedly mentioned in CSI,when the investigators go nowhere but determined to find the criminal.
The exchange principle is postulated by 20th century Edmond Locard. He was a legend. His techniques were instrumental to the French secret service during World War I.
Here is the excerpt from his publication.


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.
To simplify, when 2 objects touch, there must be a material exchange between them.
The typical case is Locard found a metal piece on one victim's apparent,  and the metal was identical to the material of a coin at the crime scene. How bad made was the coin at that time! 
With the surge of criminals' talent,  their exquisite design leaves the least trace at the scene, and destroy any DNA evidence with bleach. 
Here comes a diploma: if molecule level of evidence is found, is it sufficient to be a legal proof to point out the criminal? because the exchange may happen before the crime or be caused by contamination in the lab.