Table

Widths of grooves

(Measurements made with the filar micrometer)

Grooves on fatal bullets

Grooves on test bullets

Bullet X

Bullet Y

Bullet 1

Bullet 2

Bullet 3

.038 in.

.038 in.

.038 in.

.038 in.

.038 in.

.031 „

.031 „

.031 „

.031 „

.031 „

.034 „

.034 „

033 „

033 „

033 „

.031 „

.031 „

.032 „

.031 „

.032 „

.032 „

.032 „

.032 „

.031 „

.032 „

033 „

033 „

033 „

033 „

033 „

Note: The technique illustrated in this table is not only outmoded, because of the adoption of the comparison microscope which affords a much more positive means of identification, but would be inapplicable in these days when so many barrels are rifled by either the broaching or the button-swaging process. Barrels rifled successively with the same broach (or button) will have rifling grooves (and lands) which will show the same sequence of widths, thus making it impossible to say which of several barrels (rifled with the same tool) was the one from which a particular bullet was fired.

Note: The technique illustrated in this table is not only outmoded, because of the adoption of the comparison microscope which affords a much more positive means of identification, but would be inapplicable in these days when so many barrels are rifled by either the broaching or the button-swaging process. Barrels rifled successively with the same broach (or button) will have rifling grooves (and lands) which will show the same sequence of widths, thus making it impossible to say which of several barrels (rifled with the same tool) was the one from which a particular bullet was fired.

Two fatal bullets and a revolver taken from the suspect were submitted for examination. Three test bullets were fired into cotton waste, and the grooves on all five bullets were measured. All the bullets were very unusual as to the variation of the widths of the rifling grooves-which was an additional point in favor of the identification. The suspect confessed. Naturally this method cannot be used unless a sequence of measurements can be made, and often one does not have such a sequence because the bullet may be deformed too much.

Another method of identification used in the precomparison-microscope days was known as the method of interchange, which apparently originated in France-a method which required much skill, patience, and time. The evidence bullet was set up in front of a long-focus camera provided with a short-focus lens in order to get good magnification. Illumination was adjusted so as to bring out to best advantage the details of the markings on the bullet. If the bullet had six grooves, six pictures were taken, in sequence. The bullet was rotated and carefully positioned so that each succeeding groove occupied the same position on the ground glass as the preceding one. It is important that the angle of view be the same. Usually the groove, being the bearer of the best markings, occupied the exact center of the ground glass. Once established, the illumination was kept the same for all six exposures. Then the evidence bullet was replaced by a test bullet and a sequence of six pictures was made in the same manner, using the same illumination throughout. After prints were made from the properly numbered negatives, sections of the test pictures were cut out and placed on the appropriate evidence pictures to see if the markings matched. If portions of two photos (one of the evidence bullet, the other of a test bullet) when placed in juxtaposition were found to have a sufficient number of lines which were continuous across the boundary, the pictures were said to be matched.

In addition to being time consuming and tricky the method has the disadvantage that one never knows what degree of success he has had until all the pictures are taken and compared. Unless the bullets are placed in exactly the right position when photographed, the results will be disappointing. Figs. 14 and 15 show an application of this „method of interchange" as applied to the identification of shell heads through a comparison of breechblock markings. The method is much easier to apply in the case of shell heads because a whole series of pictures is not needed. It is of course necessary that the shells be placed in the same position and that they be illuminated the same, from the same direction and angle. Some experimentation will be necessary to determine the best angle for the illumination. Markings are made more distinct by using a low angle of illumination at right angles to scratches that may be present on the object being photographed.

Modern comparisons are usually made by the comparison microscope which for many years has been the standard method. This instrument and the comparison camera will be described in a later section. In these instruments portions of each object being examined can be brought into juxtaposition either in the optical field of the microscope or on the ground glass of a specially constructed camera and no pictures are made until a matching of characteristic features in the two areas under observation is found.

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