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I've been following your history. It looks like you're in a forensic science class. ~~~~~~~~~~~ A comparison microscope is a microscope that lets us examine two objects, at the same time. We can use it to do side-by-side comparisons of two different pieces of evidence. Here's a picture of a comparison microscope: http://www.proway-microscope.com/products_img/20090215210818.jpg You see, it's basically two microscopes, joined together. There's a LEFT microscope, joined to a RIGHT microscope. When we look into the eyepiece, what we see is one large circle, cut in half. Like this: http://i769.photobucket.com/albums/xx336/Cardinal25/ComparisonImagers020.jpg The LEFT half of the circle shows us the object that's placed under the LEFT microscope. And the RIGHT half of the circle shows us the object that's placed under the RIGHT microscope. ~~~~~~~~~~~~~~~ Comparison microscopes, again, let us compare two objects, at the same time. And these microscopes are most often used to compare two things: 1. Tool marks 2. Ballistic evidence ~~~~~~~~~~ TOOL MARKS are marks that tools leave behind, on solid surfaces. One example is the scrape mark that a crowbar leaves, on a wooden window seal, as someone is trying to break into a house. If we look real closely, under a microscope, we can see that different tools leave unique marks. If you and I were both using crowbars, to break into a house, my crowbar would not leave behind the same scratch marks, on the wood, as your crowbar would. The marks left behind by each crowbar would be different. Unique. ~~~~~~~~~~~~~ So, imagine that we're CSI. We get to a burglary crime scene. We see that the burglar used some kind of tool, to pry open the door, in order to get inside the house. Whatever he used, it left behind scratch marks, on the door frame. What we can do is cut out, and bring a whole section of the door frame back with us, to the crime lab. So now, we have a section of the wooden door frame, that has the tool marks engraved on it. Later, we search the house of a guy, who we suspect is the burglar. Inside his house, we find a crowbar. It looks like it's been used. We can confiscate the crowbar, and take it with us back to the lab. Back at the lab, we now have a door frame section, with the tool marks, AND a crowbar, taken from the suspect. We can use this crowbar, and make an intentional scratch mark on another piece of wood, from the same door frame. One piece of wood has the original tool mark, and the other piece of wood now has the tool mark that we intentionally created. Now, we can examine both of these pieces of wood, under a comparison microscope. If the scratch marks are identical, then we know that this crowbar, that we have, was used to break into the house. You see? And we can only tell whether they are identical, by looking at them side by side, under a comparison microscope. ~~~~~~~~~~~~~~~ BALLISTICS EVIDENCE is also often examined under a comparison microscope. Here's a fact you may already know: When two (or more) bullets are fired from the same gun, they will end up with the same STRIATIONS (scratch marks). This is because when a bullet is fired from a gun, it gets little tiny scratches on it, while it's exiting the gun barrel. ~~~~~~~~~~~~~~~ So, once again, let's imagine that we are CSI. We get to a murder crime scene. Some guy's been shot to death. And the bullets are still in his body. We can eventually get our hands on one of the bullets, and bring it back with us, to the crime lab. Later, we search the house of the guy, who we believe is the murderer. Inside his house, we find a gun. What we can do is take his gun with us, back to the crime lab. At the crime lab, we can use the gun in order to fire a bullet. We take the bullet that we fired, and we compare it with the bullet from the crime scene, under a comparison microscope. If the STRIATIONS from both of these bullets match, then we know that this gun, that we have, was the murder weapon. You get it? ~~~~~~~~~~~~~~~~ Once again, comparison microscopes allow us to look at two objects, at the same time, and compare their microscopic details. This is useful when we're looking at evidence that needs to be accurately compared, such as TOOL MARKS, and BALLISTIC EVIDENCE.