Comparison Microscope

A comparison microscope is a device used to analyze side-by-side specimens. It consists of two microscopes connected by an optical bridge, which results in a split view window enabling two separate objects to be viewed simultaneously. This avoids the observer having to rely on memory when comparing two objects under a conventional microscope.

In the 1920s forensic ballistics was waiting at its inception. In 1929, using a comparison microscope adapted for the purpose by Calvin Goddard and his partner Phillip Gravelle used similar techniques to absolve the Chicago Police Department of participation in the St. Valentine's Day Massacre.

Philip O. Gravelle, a chemist, developed a comparison microscope for use in the identification of fired bullets and cartridge cases with the support and guidance of forensic ballistics pioneer Calvin Goddard. It was a significant advance in the science of firearms identification in forensic science. The firearm from which a bullet or cartridge case has been fired is identified by the comparison of the unique striae left on the bullet or cartridge case from the worn, machined metal of the barrel, breach block, extractor, or firing pin in the gun. It was Gravelle who mistrusted his memory. "As long as he could inspect only one bullet at a time with his microscope, and had to keep the picture of it in his memory until he placed the comparison bullet under the microscope, scientific precision could not be attained. He therefore developed the comparison microscope and Goddard made it work." Calvin Goddard perfected the comparison microscope and subsequently popularized its use.Sir Sydney Smith also appreciated the idea, emphasizing its importance in forensic science and firearms identification. He took the comparison microscope to Scotland and introduced it to the European scientists for firearms identification and other forensic science needs.

The modern instrument has many optical, mechanical and electronic refinements, including fiber optic illumination, video capabilities, digital imaging, automatic exposure for conventional photography, etc. Despite this evolution, however, the basic tools and techniques have remained unchanged which are to determine whether or not ammunition components were fired by a single firearm based on unique and reproducible microscopic and class characteristics, or to reach a "no conclusion" result if insufficient marks are present.

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