The bullets can be seen on their holders, and the two projeccion lamps are attached to the fronts of the stages focussed with the microscope. Any markings along that part of the edge will then be seen distinctly. The case is then rotated so that every part of the side can be examined in turn.
I have purposely explained the procedure of examination with some detail because the instrument usually adopted and recommended for this work is that known as a "Comparison Microscope." Such an instrument really consists of two microscopes, optically paired, and mounted parallel one to the other. One cartridge to be examined is put under one of these microscopes, and another under the other. The two microscopes are connected by a special eye-piece which enables the observer to view both cartridge cases, or portions of both cases, in the same field of view at the same time. He is thus enabled to compare the marks on the two cases just as one can compare two photographs which are placed side by side more easily than when they are seen one after the other.
The most fantastic claims have been put forward for, and the most ridiculous descriptions of, this type of instrument which are enough to suggest that it has magical properties, and that it automatically, and wholly of its own accord, rings a bell or utters some similar warning, when the two cartridge cases under examination exhibit the same thumb-mark. Unhappily there is no foundation for this comforting belief. In the hands of a trained microscopist the comparison microscope can be of great value in determining the identity of fired bullets; but for cartridge cases I have come to the conclusion that a high-class single instrument is preferable. After all, the sole advantage of the comparison microscope is that it enables one to see two objects in the same field. But this can be done, as I have explained, quite easily with an ordinary microscope; and after exhaustive trial I have now given up my comparison microscope entirely for cartridge cases, keeping it only for the examination of fired bullets.
And even for fired bullets the comparison microscope offers difficulties in illumination which are never encountered when using a single instrument, for the illumination of opaque objects such as the surfaces of fired cartridge cases and bullets is a far more difficult problem than the illumination of transparencies, such as blood smears, fibres or spermatazoa. In fact in the case of transparencies, illumination is really a comparatively simple matter until a magnification of about X 300 is exceeded, and for transparent objects examined at low powers, such as textiles, the comparison microscope is ideal. But the oblique illumination of opaque surfaces can provide so many pitfalls that no one but a trained microscopist should be asked to make the examinations.
For cartridge cases I use a "Horizontal" microscope by Messrs. W. Watson. This form of instrument was originally designed for metallurgical work in the form of the Zeiss-Martens stand, and is peculiarly suited for examining cartridge cases and the breech faces of weapons, as it is fitted with both body and stage focussing, and there is an altogether exceptional clearance between stage and objective.
It is also an ideal instrument for photo-micrography, which must always play a very important part in the task of identifying firearms, as without good photographs evidence of identification must consist merely of statements of opinion, when its value is not great.
So far I have confined myself to what may be termed positive evidence of identification, that is the finding of the thumb-mark of the suspect pistol on the "crime" cartridge.
This fact in itself may not necessarily be sufficient definitely to marry pistol and cartridge, because although it is true that every breech face has an individuality of its own, it is also a fact that all cuts made by the same tool in a machine will bear a strong "family likeness" to each other. And since brass is not a perfectly plastic substance, such as warm wax, it may happen that the cartridge case is only imprinted with the "family" thumb-mark common to every breech face of that particular batch, and not with the individual thumb-mark of the breech face of one single pistol1
A rather low pressure, for instance, may easily result in the cartridge being imprinted only with some "family" mark instead of the smaller individual marks of one pistol. Similarly the brass used in different makes of cartridges, as well as the metal used in the caps, may vary in hardness and so vary in their readiness to receive a clear imprint of the breech face. As a matter of fact cartridge brass is nearly always 70 per cent, copper and 30 per cent, zinc, although some makers adopt 71 and 29 per cent, while others adopt 69 and 31 per cent. But they all keep near the 70 and 30 per cent. The metal used in caps, however, does vary in its susceptibility to receive imprints, and for this reason the test cartridges should always, whenever possible, be of the same make as the "crime" cartridge.
This is the theoretical aspect of the problem which, at first sight, may appear to be overwhelming. I have tried to confirm or refute it for the past twenty years,
1 The existence of a ''family likeness" between different cuts with the same tool has very properly been used by the Crown in a prosecution for murder. Evidence was given that the surfaces of two severed telephone wires showed the same characteristics when viewed under the microscope as the severed surfaces of similar wire which was cut for test purposes, and from this it was deduced that the knife which was used for cutting the test wire had also been used for cutting the original wires.
and I have personally reached the conclusion that in actual practice theory is not borne out. This is especially the case with what may be termed medium and high grade weapons, including most firearms made to government specifications in almost all countries. The highest grade weapons of this general type will always have their breech faces finished by subsequent grinding, or even filing. Even if this work is most cursory it cannot fail to establish a new set of markings which must be individual for every breech face treated. For the chances of grinding or file marks being absolutely identical in two or more breech faces are so remote as to be hardly worth consideration. And even lower grade weapons are almost bound to have some sort of finishing which will tend to change the family likeness in every case.
The chief risk connected with the family likeness lies in the original tool markings only being partially obliterated by subsequent work, and when this occurs it is possible to mistake some very pronounced mark or marks for the one and only "thumb-mark" of some particular weapon. Such pronounced marks are easily seen and easily photographed and tend to attract attention away from the more insignificant, finer, and less visible tool-marks left by the work subsequent to the original cuts. It is these finer markings which are of primary and vital importance, and any identification based solely on one or two major markings without any finer striations as well should be regarded with suspicion.
So the possibility of the existence of a family likeness or thumb-mark must be kept in mind, and when some very pronounced and obvious tool cuts have been found to leave their imprint on the base of a fired cartridge the investigator should not jump at conclusions too rapidly, but should search carefully for some finer imprints which
Two Different Bullets which had been fired by the same Revolver as viewed under the two microscopes of the Forensic Microscope shown in Plate XXIX
The ordinary eye-pieces are in use and the two bullets are viewed separately one after the other will possibly be of more value in determining the true thumb-mark of the weapon which fired the cartridge.
And when identification is based on comparatively vague generalities of major markings, as it sometimes is although it never should be, negative evidence becomes essential and without it the positive evidence is completely valueless. By this I mean that before it is possible to declare that the "crime" cartridge marries the suspect weapon it is essential to prove that it does not marry equally well any single one of a number of similar weapons of the same make.
Although many years of constant, careful, and exhaustive examinations of literally thousands of fired cartridge cases and very large numbers of breech faces of many different types of weapons from high grade revolvers to Sten guns and best quality rifles to the more mass-produced service rifles of the 1940 to 1945 period, have convinced me that every breech face has its own separate and distinct individuality, I do believe that there is a risk of this true individuality not always being recognised. It is so easy to be fascinated by some single and very pronounced tool-mark which may be, and quite probably is, the most important part of the thumb-mark of some particular weapon. But a more careful examination, with possibly a slightly different angle of illumination, will always discover some finer markings, and it will be the combination of these finer markings with the more startling fascinator which will provide the true individuality of the weapon.
Some time ago I was engaged in a case which was heard overseas and in which the Crown expert based his identification of two fired cartridge cases found on the scene of a crime with a self-loading pistol found in the possession of the accused on the following: "identity" of ejector marks; "identity" of cap indentations; a single scratch on the upper part of the side of each cartridge.
What he meant by "identity" was never quite clear because he declared that there were no imprints of tool markings of the striker and breech face on either of the "crime" cartridges. And this in spite of the fact that the pistol was a cheap machine-finished weapon.
The photographs of the cartridge bases produced by the prosecution were taken with an ordinary camera and showed nothing beyond a vague similarity of ejector marks in the "crime" and test cases. But although different makes of pistol will give very different ejector marks, and although even different batches of one make of pistol will produce different ejector marks, as is shown in Plate XVIII (a) and (b), there are almost bound to be a number of pistols of that make, even if not of other makes as well, which will produce quite similar ejector marks. And merely somewhat similar ejector marks by themselves are of no serious value in establishing true identification.
The "identity" of the striker indentations was supported by the crudest attempt at photo-micrography which I have ever seen except one—and that was the photo-micrograph taken by the same expert in the same case of the "crime" bullet. This last photograph was merely a dark smudge, while the one of the striker indentations was a vague blur in which not one vestige of detail was visible.
The "scratch evidence" was that to which I have already referred on page 123.
Good photographs are an absolutely essential part of the evidence for the prosecution which is based on the identification of the accused's weapon with the "crime" cartridge. These photographs need not necessarily be taken at a high magnification—5 diameters is usually ample—but it is important that the photograph of the "crime" cartridge and those of all the test cartridges should be taken with the source of illumination at exactly the same angle in every instance, as a variation in the angle of lighting can easily render some important mark invisible.
If the identification is based on the smaller striations on the cap "composite" photographs taken at a higher magnification can provide most convincing evidence, especially to a non-technical jury.
An example of "composite" photographs is given in Plate XXV. The two left-hand photographs show the caps of the two '303 cartridges which comprise the bottom pair in Plate XIX, but it will be seen that the magnification is distinctly higher. The right-hand pair of photographs is made up with the central part of one cap cut out and inserted over the other cap. But in spite of this "patchwork" arrangement this photograph looks but little different from the other ones, while the various markings fit in perfectly. These "composite" photographs are largely used in America where evidence of the identification of firearms is far more common than in this country.
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