# Comparison microscope

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Comparison_microscope
> Markdown URL: https://mediated.wiki/source/Comparison_microscope.md
> Source: https://en.wikipedia.org/wiki/Comparison_microscope
> Source revision: 1337526707
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

Type of microscope

In a comparison microscope, two identical microscopes are connected to a single comparison eyepiece. The viewer sees the images from both microscopes next to one another, as in the inset image.

A **comparison microscope** is a device used to analyze side-by-side specimens. It consists of two [microscopes](/source/Microscope) 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.

## History

One of the first prototypes of a comparison microscope was developed in 1913 in Germany.[1]

In 1929, using a comparison microscope adapted for forensic ballistics, Calvin Goddard and his partner Philip Gravelle were able to absolve the Chicago Police Department of participation in the St. Valentine's Day Massacre.

### Col. Calvin H. Goddard

Main article: [Calvin Hooker Goddard](/source/Calvin_Hooker_Goddard)

Philip O. Gravelle, a [chemist](/source/Chemist), developed a comparison microscope for use in the identification of fired [bullets](/source/Bullets) and [cartridge cases](/source/Casing_(ammunition)) with the support and guidance of forensic ballistics pioneer Calvin Goddard. It was a significant advance in the science of [firearms](/source/Firearms) identification in [forensic science](/source/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](/source/Gun_barrel), breach block, [extractor](/source/Extractor_(firearms)), or [firing pin](/source/Firing_pin) in the [gun](/source/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.[2] [Sir Sydney Smith](/source/Sydney_Smith_(forensic_expert)) also appreciated the idea, emphasizing its importance in forensic science and firearms identification. He took the comparison microscope to [Scotland](/source/Scotland) and introduced it to the European scientists for firearms identification and other forensic science needs.

## Modern comparison microscope

The modern instrument has many [optical](/source/Optical), [mechanical](/source/Machine) and [electronic](/source/Electronics) refinements, including [fiber optic](/source/Fiber_optic) illumination, [video](/source/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](/source/Firearm) based on unique and reproducible microscopic and class characteristics, or to reach a "no conclusion" result if insufficient marks are present.[3]

Since, ballistic identification has benefited from a long series of structural, scientific and technological advances, law enforcement agencies have established forensic laboratories and researchers have learned much more about how to match [bullets](/source/Bullet) and [cartridge cases](/source/Casing_(ammunition)) to the guns used to fire them, and comparison microscopes have become more sophisticated. By the end of the 1980s, ballistic identification was an established sub-specialty of [forensic science](/source/Forensic_science).

Visualization tools have also been developed to allow the firearms examiner to verify the degree of similarity between any two tool-marks in question. These are designed to simulate the operation of the comparison microscope but are capable of rendering a 2D view of the 3D surfaces in a manner similar to that of the conventional comparison microscope.

## Forensic ballistics

Main article: [Ballistics](/source/Ballistics)

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources in this section. Unsourced material may be challenged and removed. (May 2022) (Learn how and when to remove this message)

As with most firearms, the fired [ammunition](/source/Ammunition) components may acquire sufficient unique and reproducible microscopic marks to be identifiable as having been fired by a single firearm. Making these comparisons is correctly referred to as firearms identification, or sometimes called as "ballistics".

Historically, and currently, this [forensic discipline](/source/Forensic_ballistics) ultimately requires a microscopic side-by-side comparison of fired [bullets](/source/Bullet) or [cartridge cases](/source/Casing_(ammunition)), one pair at a time, by a forensic examiner to confirm or eliminate the two items as having been fired by a single firearm. For this purpose, the traditional tool of the firearms examiner has been what is often called the ballistics comparison microscope.

The interior of a gun's barrel is machined to have grooves (called [rifling](/source/Rifling#Manufacture)) that force the bullet to rotate as it travels along it. These grooves and their counterpart, called "lands" imprint groove and land impressions on the surface of the bullet. Together with these land and groove impressions, imperfections on the [barrel](/source/Gun_barrel) surface are incidentally transferred to the bullet's surface. Because these imperfections are randomly generated, during manufacture or due to use, they are unique to each barrel. These patterns or imperfections, therefore, amount to a "signature" that each barrel imprints on each of the bullets fired through it. It is this "signature" on the bullets imparted due to the unique imperfections on the barrel that enable the validation and identification of bullets as having originated from a particular gun. Comparison microscope is used to analyze the matching of the microscopic impressions found on the surface of bullets and casings.

When a firearm or a bullet or cartridge case are recovered from a [crime scene](/source/Crime_scene), forensic examiners compare the [ballistic fingerprint](/source/Ballistic_fingerprinting) of the recovered bullet or cartridge case with the ballistic fingerprint of a second bullet or cartridge case test-fired from the recovered firearm. If the ballistic fingerprint on the test-fired bullet or cartridge case matches the ballistic fingerprint on the recovered bullet or cartridge case, investigators know that the recovered bullet or cartridge case was also fired from the recovered gun. A confirmed link between a specific firearm and a bullet or cartridge case recovered from a crime scene constitutes a valuable lead, because investigators may be able to connect the firearm to a person, who may then become either a suspect or a source of information helpful to the investigation.

## Notable cases

### Sacco and Vanzetti case

Main article: [Sacco and Vanzetti](/source/Sacco_and_Vanzetti)

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources in this section. Unsourced material may be challenged and removed. (May 2022) (Learn how and when to remove this message)

Forensic innovator [Calvin Goddard](/source/Calvin_Hooker_Goddard) offered ballistic identification evidence in 1921 to help secure [convictions](/source/Conviction) of accused murderers and anarchists [Nicola Sacco](/source/Nicola_Sacco) and [Bartolomeo Vanzetti](/source/Bartolomeo_Vanzetti). On April 8, 1927, Sacco and Vanzetti were finally sentenced to death in the [electric chair](/source/Electric_chair). A worldwide outcry arose and Governor Alvin T. Fuller finally agreed to postpone the [executions](/source/Capital_punishment) and set up a committee to reconsider the case. By this time, firearms examination had improved considerably, and it was now known that a [semi-automatic pistol](/source/Semi-automatic_pistol) could be traced by several different methods if both [bullet](/source/Bullet) and [casing](/source/Casing_(ammunition)) were recovered from the scene. Automatic pistols could now be traced by unique markings of the rifling on the bullet, by [firing pin](/source/Firing_pin) indentations on the fired primer, or by unique ejector and [extractor](/source/Extractor_(firearms)) marks on the casing. The committee appointed to review the case used the services of [Calvin Goddard](/source/Calvin_Hooker_Goddard) in 1927. Goddard used Philip Gravelle's newly invented comparison microscope and helixometer, a hollow, lighted magnifier probe used to inspect [gun barrels](/source/Gun_barrel), to make an examination of Sacco's .32 Colt, the bullet that killed Berardelli, and the spent casings recovered from the scene of the crime. In the presence of one of the defense experts, he fired a bullet from Sacco's gun into a wad of cotton and then put the ejected casing on the comparison microscope next to casings found at the scene. Then he looked at them carefully. The first two casings from the robbery did not match Sacco's gun, but the third one did. Even the defense expert agreed that the two cartridges had been fired from the same gun. The second original defense expert also concurred. The committee upheld the convictions. In October 1961, ballistics tests were run with improved technology using Sacco's Colt automatic. The results confirmed that the bullet that killed the victim, Berardelli in 1920 came from the same .32 Colt Auto taken from the pistol in Sacco's possession. Subsequent investigations in 1983 also supported Goddard's findings.

### St. Valentine's Day massacre

Main article: [St. Valentine's Day Massacre](/source/St._Valentine's_Day_Massacre)

Colonel Goddard was the key forensic expert in solving the 1929 [St. Valentine's Day Massacre](/source/St._Valentine's_Day_Massacre) in which seven gangsters were killed by rival [Al Capone](/source/Al_Capone) [mobsters](/source/Mobster) dressed as [Chicago police](/source/Chicago_Police_Department) officers. It also led to the establishment of the United States' first independent criminological laboratory, which was located at [Northwestern University](/source/Northwestern_University) and headed by Goddard. At this new lab, [ballistics](/source/Ballistics), [fingerprinting](/source/Fingerprinting), blood analysis and [trace evidence](/source/Trace_evidence) were all brought under one roof. In 1929, using a comparison microscope adapted for the ballistics comparison by his partner, Phillip Gravelle, Goddard used similar techniques to absolve the Chicago Police Department of participation in the [St. Valentine's Day Massacre](/source/St._Valentine's_Day_Massacre). The case of Sacco and Vanzetti, which took place in Bridgewater, Massachusetts, is responsible for popularizing the use of the comparison microscope for bullet comparison. Forensic expert Calvin Goddard's conclusions were upheld when the evidence was re-examined in 1961.

## References

1. **[^](#cite_ref-1)** ["A Double Microscope with One Eyepiece"](https://babel.hathitrust.org/cgi/pt?id=wu.89046350930&view=1up&seq=645). *Popular Mechanics Magazine*. **19**: 642. May 1913 – via HathiTrust.

1. **[^](#cite_ref-2)** Jurgen Thorwald, **The Century of the Detective**, New York: Harcourt, Brace & World, 1964

1. **[^](#cite_ref-dillon_3-0)** John H. Dillon, Jr, Comparison Microscopy: The Origins of Firearms identification, A Practical Application of Forensic Science, Technology and Engineering to Case Linkage in Shooting Incidents Not Previously Related by Investigative personnel, BulletTRAX-3D, MatchPoint Plus and the Firearms Examiner, 2005.

## External links

- [Seibert Wetzlar Vergleichsmikroskop](http://www.musoptin.com/seibert_15368.html)

- [Comparison microscopes](https://www.google.com/images?hl=en&q=comparison%20microscopes&um=1&ie=UTF-8&source=og&sa=N&tab=wi)

- [Virtual Comparison Microscope](http://www.firearmsid.com)

## See also

- [Association of Firearm and Tool Mark Examiners](/source/Association_of_Firearm_and_Tool_Mark_Examiners)

- [Rifling](/source/Rifling)

v t e Laboratory equipment General Heaters Dryers Alcohol burner Bunsen burner Desiccator Heating mantle Hot plate Lab oven Kiln Meker–Fisher burner Striker Teclu burner Water bath Vacuum dry box Mixers Shakers Chemostat Homogenizer Liquid whistle Magnetic stirrer Mortar and pestle Shaker Sonicator Static mixer Stirring rod Vortex mixer Wash bottle Stands Clamps Holders Beaker clamp Clamp holder Tripod Burette clamp Extension clamp Flask clamp Funnel support Iron ring Pinch clamp Retort stand Screw clamp Test tube holder Test tube rack Wire gauze Lab drying rack Containers Storage Agar plate Cryogenic storage dewar Incubator Laminar flow cabinet Microtiter plate Petri dish Picotiter plate Refrigerator Weighing boat Weighing dish Other items Aspirator Autoclave Balance brush Cork borer Crucible Filter paper File Forceps Centrifuge Microscope Pipeclay triangle Spectrophotometer Splint Stopper Scoopula Spatula Test tube brush Wire brush Inoculation needle Inoculation loop Glassware Apparatus Dean–Stark Soxhlet extractor Kipp's Bottles Boston round Condensers Cold finger Liebig Dishes Evaporating Petri Syracuse Watch glass Flasks Büchner Vacuum (Dewar) Erlenmeyer Fernbach Fleaker Florence Retort Round-bottom Schlenk Volumetric Funnels Büchner Hirsch Dropping Separatory Measuring devices Burette Conical measure Cuvette Eye dropper Eudiometer Graduated cylinder Ostwald viscometer Pipette Tubes Drying Cragie Nuclear magnetic resonance (NMR) Test Thiele Thistle Other items Beaker Bell jar Gas syringe Vial Analytical chemistry Compositional AutoAnalyzer CHN analyzer Colorimeter Inductively coupled plasma (ICP) device Gas chromatograph (GC) Liquid chromatograph (LC) Mass spectrometer (MS) pH indicator pH meter Microscopy Scanning electron microscope (SEM) Transmission electron microscope (TEM) Thermochemistry Calorimeter differential scanning Melting-point apparatus Thermometer Thermogravimetric analyzer (TGA) Other items Analytical balance Colony counter Spiral plater Nuclear magnetic resonance (NMR) instrument Plate reader Electronics Control devices Bench power supply Current source Voltage source Function generator Galvanostat Pulse generator Potentiostat Measurement Ammeter Logic analyzer Multimeter Network analyzer Oscilloscope Spectrum analyzer Time-domain reflectometer Transistor tester Voltmeter Tools Heat gun Soldering iron Tweezers Wire stripper General Alligator clip Test probe Safety Personal protective equipment (PPE) Lab coat Face shield Respirator Rubber apron Safety shower Eye and hand Acid-resistant gloves Eyewash station Glove box Medical gloves Nitrile gloves Safety glasses Safety goggles Other items Biosafety cabinet Fire blanket Fire extinguisher Fume hood Safety cabinet Solvent cabinet Instruments used in medical laboratories

---
Adapted from the Wikipedia article [Comparison microscope](https://en.wikipedia.org/wiki/Comparison_microscope) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Comparison_microscope?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.