READ: Trace Evidence and Microscopes

Comparison Microscope

 The comparison microscope is another essential tool in the crime lab. It allows for comparison of various artifacts of evidence simultaneously and is the primary microscopy tool used in trace evidence investigations of bullet, hair, fiber, and tool mark evidence. It was developed by Phillip Gravelle in the 1920s and first used by Calvin Goddard to examine evidence. In 1927, Goddard was called to examine evidence from the Sacco and Vanzetti case due to a huge outcry from the public at the time of their execution. Goddard was able to ascertain that the bullet that killed the victim was fired from Sacco's gun by using the comparison microscope to compare the spent bullet casings. Goddard was called to analyze bullet evidence and testify in another famous case involving the comparison microscope, the St. Valentine's Day Massacre, in 1929. He examined bullets and cartridge evidence and ultimately absolved the Police Department of Chicago from any involvement in the massacre based upon his findings using the comparison microscope.

A comparison microscope is basically two compound microscopes that can be viewed simultaneously on a split screen. The split screen view can be rotated so that the two items being compared are oriented in the same direction for easier comparison. The split screen can also be changed to an overlay setting where one field of view is placed on top of the other as an overlay to see how closely the evidence items align or confirm a match. Often each piece of evidence is given a different color filter when using the overlay technique so that the segments that overlay are darker than the areas that do not match up. The comparison microscope also ensures that the items of evidence being analyzed are viewed under the same lighting and magnification. The images from the microscope are often projected on to a television screen or computer monitor. Photo documentation of the magnified views are documented either by screenshot or directly from a camera mounted within the microscope. These images are very important documentation of what was seen at the time of microscopic analysis and are frequently displayed in court as evidence when an expert is testifying.

Georgia Virtual, Microscopes, CC BY-NC-SA 3.0