The comparison microscope

Since the comparison microscope is probably the most important and most widely used scientific instrument in the modern crime laboratory it is fitting that it be discussed first.

The first comparison} microscope to be used in the field of criminology was a very crude instrument compared to the highly developed instruments now available. It was designed by Albert S. Osborn for application in the field of document examination and was constructed by the Bausch and Lomb Optical Company. As a matter of fact, Osborn's first idea was to use it for the comparison of the colors (and particularly the ageing) of documents, and it was built to accommodate the filters of the Lovibond Tintometer. But he soon found it to be very useful for other comparisons as well. This instrument is shown in Fig. 45. * There is no hint that it was ever used in the field of firearms identification.

*A. S. Osborn-Questioned Documents, 1st Ed. 1910, page 326.

In 1922 C. E. Waite, a special investigator, was assigned by the Governor of New York to a homicide case in which a firearm played a part, and he thus became interested in the possibility of the identification of firearms. But his ideas on the subject were very vague indeed. He gathered much information relative to existing firearms, their specifications and manufacture, but was greatly handicapped by the fact that he had no scientific training nor mechanical background and no particular knowledge of firearms. But he was a promoter and he had an idea. So he formed a group of associates to supply what he himself lacked, and this organization later took the name „The Bureau of Forensic Ballistics." His associates were Philip O. Gravelle, a photographer and microscopist, John H. Fisher, an expert tool designer and worker with precision instruments, formerly at the Bureau of Standards, and (most important of all) Captain Calvin Goddard who had served in World War I as a Medical Officer, whose hobby was guns of all descriptions, and who had a very unusual knowledge of firearms, their construction, manufacture, types and makes, specific models, individual characteristics, and peculiarities. Little did he know at the time that he was destined to become the pioneer and most distinguished worker in the field of firearms identification. Fisher's chief contribution, and the only one of record, was the design of an instrument to which was given the name Helixometer, the purpose of which was to permit observations of the interior of a barrel and measurements of the pitch of the rifling to be made. The instrument was made for a number of years by the Spencer Lens Co., but it was not a success and those in existence are rarely, if ever, used.

To Gravelle must be given the credit for suggesting the use of a comparison microscope in firearms identification. He had been employed in pattern designing for a textile company and was familiar with the use of an instrument of this sort for comparing the fine details of two similar pieces of cloth, and early in 1925 he suggested to the group that a comparison microscope might be useful for making comparisons of fired bullets. Apparently, neither Gravelle nor Goddard knew that Professor E. M. Chamot of Cornell University had used a comparison microscope made by Bausch and Lomb in his studies on primers for the Ordnance Department during World War I. This instrument was a great improvement over that used by Osborn, particularly as it had two mechanical stages for holding the objects being examined. The optical system was also greatly improved.

As a result of Gravelle's suggestion two compound microscopes, a comparison bridge of foreign manufacture, and a pair of bullet mounts (probably designed by Fisher) built by the Remington Arms Co., were procured and first set up for use in April 1925. It was a great success from the beginning and Goddard published a paper on the subject in the November-December issue of Army Ordnance. The instrument soon obtained wide publicity through this and other articles, and to Goddard goes the principal credit for this great advance in firearms identification technique, since Gravelle never actually used the instrument in firearms identifications.

A comparison microscope (Figs. 46 to 52) consists essentially of two compound microscopes, having identical optical systems, so that they give the same magnification, connected by an optical „bridge" containing a combination of prisms such that by viewing two separate objects (one under each microscope) through a single eyepiece the two objects may be compared by bringing the images of parts of each into juxtaposition. The „optical field" seen through the single eyepiece is a circular area divided into two parts by a thin dark line. The object under the left hand microscope (say the evidence bullet) is seen in the left half of this optical field, and the object under the right hand microscope (say a test bullet) is seen only in the right half of the field. Assuming that we have two bullets to be compared, an evidence bullet and a test bullet, the evidence bullet is placed in a „bullet mount" under the left hand microscope and adjusted so that it points directly across (i.e., at right angles to) the central dividing line and in a position such that, say, a third of the rear portion of the bullet is in view. It is now rotated in its mount (by turning the appropriate knob) until a well-defined groove (or land) comes into view and this is brought into the best possible focus by raising or lowering the microscope stage carrying the bullet mount. (All focusing is done by raising or lowering the microscope stages.) Then the test bullet is placed so that about two-thirds of the nose end of the bullet is in view (pointing in the same direction as the evidence bullet, of course) and, after lining up the edges of the test bullet with those of the evidence bullet, the test bullet is slowly rotated to bring successive grooves (and lands) into view until one is found (if any such is present) which not only has the same width, but which also has longitudinal striations or tiny groovelets (scorings) which extend across the boundary line, joining and coinciding with similar markings on the evidence bullet. These markings will be parallel to the edges of the groove. When this condition is attained the bullets are said to be „matched" and, by means of a camera placed above the instrument, a photograph of the matched bullets is taken.

The mounts for the bullets are provided with removable „studs" to the ends of which the bullets are attached by a suitable wax. Several pairs of these studs are provided, usually three. Around the periphery of each pair, indexing numbers are engraved, usually from I to 4, 5, or 6, corresponding to the number of lands and grooves most often found. After marking the bullets which have been matched, they are reset on the studs so that each of the marked grooves will be in the No. I position. The two bullets are now rotated into the No. 2 position and compared. If these grooves are found to match, the matching is photographed and this process is repeated until all the grooves (and lands) have been compared. Naturally as many matches are obtained as possible, because convincing one's self and convincing a jury „beyond all reasonable doubt" are two quite different matters. The expert must always keep in mind the fact that juries are always unpredictable! If some pairs of grooves (or lands) match and others do not, the expert must be prepared to explain why they do no .

In the majority of cases the best matches of rifling marks are obtained nearer the base end of the bullets, but this is not always the case and actually the identification expert will look for matchings from one end of the „engraved" portion of the bullets to the other to make certain that nothing is overlooked. A high magnification is not necessary and, in fact, is not desirable as it naturally limits the field of observation. The magnification should never be such that the complete width of one groove (or land) cannot be seen clearly. Bullets of different sizes may be examined advantageously with different magnifications. If the grooves are narrow and have very fine striations a higher magnification will be desirable than in the case where a groove is wide and the important striations are fewer and wider. Experiment is the best guide as to the most desirable magnification to use. The author prefers to use matched Micro Tessars as these have diaphragms to control both the depth of focus and the illumination. In examining extractor marks, ejector marks, and the bottoms of firing pin impressions, higher magnifications may be desirable than are used in examining the markings on bullets. Depth of focus is very important here, especially for curved or uneven surfaces, and a small aperture must be used in taking photographs, in order to get the desired depth and clarity in the final picture, since the lens of a camera cannot adjust itself to depth as the eye does.

The comparison microscope is very useful in matching markings on fired shells. It is important in all cases that proper illumination be used. The light must come to each of the two objects being compared at the same angle and, when pictures are taken, must be adjusted so that each object is illuminated evenly and to the same intensity. Oxidized surfaces, be they lead, copper, or brass, have lower reflecting power than clean bright surfaces, hence the importance of having the illumination adjusted properly in order that each half of the negative will have the same density. Bright metal surfaces can be made less reflecting by putting them in an oxidizing atmosphere for a time, but this procedure is legitimate only on the test bullets and shells and is not often used. The question as to the use of comparison photographs in court will be discussed elsewhere, but the author is firmly convinced that photographs should be taken as a matter of record, if for no other purpose. No man's memory of what he has seen under a microscope is infallible, particularly of such complex systems as those constituting bullet matches, and one photograph is worth hundreds of descriptive words in one's notes! A case may be appealed to a higher court or may be reopened years later. In such cases photographs taken at the time of the original investigation are very valuable indeed.

The accompanying illustrations of comparison microscope matchings (Figs. 53 to 60) show what can be done when conditions are reasonably good. It must not be supposed that convincing matchings can always be obtained. Every examiner, no matter how experienced or expert he may be, has had the experience of spending many hours in the futile attempt to get satisfactory and convincing matchings in cases where there was every reason to believe that he had the gun that fired the evidence bullet or shell. There are many causes for such failures. The evidence bullet may have been too small for the bore of the gun, the barrel may have been very rusty or fouled, the bullet may have been so deformed that no evidence of lands or grooves is present, etc. The author once received a fatal .22 cal. lead bullet that was so crushed and mutilated that it bore no rifling markings whatsoever-it was simply a „misshapen hunk of lead that weighed what a .22 cal. long rifle bullet of a certain make should weigh." (Incidentally, .22 cal. lead bullets, particularly .22 shorts, probably cause more trouble than all other bullets put together.) A spinning bullet fired into plaster may have all identifying marks removed. And primers may sometimes be so hard, or powder pressure may be so low, that no breechblock markings are produced. Many things can happen to produce non matching, but when good matchings are obtained no evidence is more convincing!

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  • Helmi
    Are bullets always mounted with the nose to the right on comparison microscope?
    8 years ago

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