A comparison microscope consists of two separate microscopes connected together and is used for comparing two objects or samples. A type of device called an optical bridge allows the user to view both samples simultaneously. This type of microscope is important in law enforcement — in the fields of criminal forensics and ballistics analysis. It allows bullets and bullet fragments as well as other forensic evidence to be compared with each other, which can be used to prove guilt or to exonerate a suspect in some crimes.
The microscope is one of the most important inventions in the history of mankind. It has allowed the advancement of many sciences and revolutionized many of our ideas about almost every physical science and even fomented the creation of new sciences. The idea of combining two microscopes to allow the study of two samples at one time was just an extension of the already well developed technologies used to build standard microscopes. The optical bridge of an early comparison microscope was nothing more than two pairs of mirrors, one for each microscope, that directed the individual images into a single viewer, producing in a side by side view of both samples in one image.
The invention of this new type of microscope resulted in the advancement of many scientific disciplines, notably, ballistics and criminal forensics. The ability to examine and compare two samples side by side made it possible to positively determine whether two bullets had been fired from one gun or from different guns. By using a test fired bullet as a control, it also made it possible to determine whether a particular bullet was fired from a certain gun or not.
Many other uses for the comparison microscope in forensics followed its invention. Samples of almost any material can be compared to establish transfer or trace evidence. Hair samples and fingerprints can also be compared in this way. Handwriting samples and biological samples, such as bacteria and algae can also be compared. Toolmarks can be compared to identify which tools were used on an object or to match a reference tool to one known to have been used in a crime.
Modern technologies have made many improvements to the basic comparison microscope. Computers and fiber optics allow the images to be displayed on monitors or projection screens and to be superimposed one over the other. Many options, like lighting, degrees of magnification, and and the ability to make photographs of sample comparisons may be available in a comparison microscope.