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The Role of the Petrographic Microscope in Criminal Investigations George T Journal of Criminal Law and Criminology Volume 31 Article 12 Issue 2 July-August Summer 1940 The Role of the Petrographic Microscope in Criminal Investigations George T. Faust Follow this and additional works at: https://scholarlycommons.law.northwestern.edu/jclc Part of the Criminal Law Commons, Criminology Commons, and the Criminology and Criminal Justice Commons Recommended Citation George T. Faust, The Role of the Petrographic Microscope in Criminal Investigations, 31 Am. Inst. Crim. L. & Criminology 246 (1940-1941) This Criminology is brought to you for free and open access by Northwestern University School of Law Scholarly Commons. It has been accepted for inclusion in Journal of Criminal Law and Criminology by an authorized editor of Northwestern University School of Law Scholarly Commons. THE ROLE OF THE PETROGRAPHIC MICROSCOPE IN CRIMINAL INVESTIGATIONS George T. Fust* There are many instances when it is objective. The purpose of the lower not possible to completely solve prob- one is to plane polarize the light which lems involving the analysis of sub- passes through the microscope. 2 The stances by either chemical or spectro- second prism, in combination with the graphic means. In such cases the de- polarizer, is utilized to determine the termination of the index of refraction optical properties of the crystal being and other optical properties by means studied.' of the petrographic microscope may While the applications of the petro- prove of value. (The index of refrac- graphic microscope are extensive, prob- tion is an optical property character- ably its most frequent use is found in istic of any particular crystalline sub- studies of the refractive index of glass stance, and it is measured by the ve- fragments picked up at crime scenes, locity with which light is transmitted and of abrasives which have been used through that substance, as compared in sabotage cases. with the velocity of light in a vacuum.) This type of microscope is designed Case musrations for the specific purpose of studying the In a certain hit-and-run case 4 sam- optical properties of crystals. The prin- ples of a broken headlight lens and one cipal difference between its construc- lens of a pair of colored sun glasses tion and that of the ordinary compound were picked up at the scene of the microscope is that in the petrographic accident. These samples, together with microscope there are two Nicol prisms,' fragments of a headlight lens and color- one (known as the polarizer) be- ed sun glasses (removed from the low the object stage and the second running board of the suspected auto- (known as the analyzer) above the mobile) were submitted for study. Both * Formerly Assistant Professor, Rutgers Uni- ever, are not in a fixed position but constantly versity, at which institution the work described vary. In the case of plane polarized light the in this paper was carried on. (Present address, planes of vibration in which the wave impulses 701-13th Street, Tuscaloosa, Alabama.) travel remain in a fixed position parallel to one 1 A Nicol prism consists of natural, clear, trans- another. parent calcite (Iceland or double spar), which 3 For a concise description of the petrographic has been cut and polished in a definite orien- microscope and its use see: Kraus, E. H., Hunt, tation. W. F., and Ramsdell, L. S., Mineralogy (3d ed.), 2 Ordinary or non-polarized light consists of a Chap. XI: 114-142 (1936). series of wave impulses which are vibrating in 4 The information regarding this case was a plane normal to the direction of transmission called to the attention of the author by Dr. H. F. or propagation. These planes of vibration, how- Vieweg in the course of personal correspondence. [ 246 ] CRIMINAL INVESTIGATIONS fragments of headlight lens (i.e., the this instance, of course, the *petro- piece picked up at the scene of the grapher could make no decision of crime and the piece taken from the legal value based solely on refractive running board) had the same refrac- index. However, another microscope tive index value (1.507) and the same study may be resorted to: namely, the specific gravity (2.457). The two sun- measurement of the amount of light glass fragments likewise had identical transmission of the glass.e These data, refractive indices (1.521), and their together with thickness measurements, specific gravities agreed within the enable the petrographer to elim- limit of error (2.517 and 2.518, respec- inate the extraneous fragments and tively). The results of this comparison thus to reconstruct a portion of the afforded a very strong indication, there- questionable window glass in jig-saw fore, that the various glass specimens fashion. original were probably from the same In a case of suspected sabotage in source. which a brake shoe, a drum, and a Research, especially with various cable were damaged, a portion of the types of glass, indicates the danger of shoe and some of the cuttings were forming too hasty a decision based submitted for study. It was suspected solely upon the index of refraction.' that someone had thrown silicon car- There are times when it will yield a bide between the shoe and the cable. definite answer, as where there is a The loose cuttings were found to be difference in refractive index, but in coated with carbonaceous material, so other instances-as in the following a portion of the sample was shaken with described case-it is advisable to make carbon tetrachloride to clean the sur- other examinations as well. faces of the larger grains. The solu- The author had occasion several tion was filtered off and the residue years ago to compare fragments of was separated into a magnetic and non- glass from a broken automobile win- magnetic portion. The magnetic mate- dow with those picked up along the rial, abraded from the cable, constitu- highway. The results of refractive in- ted about 90% by weight of the sample. dex measurements on all the specimens The non-magnetic material consisted submitted, including several obviously principally of blue fragments, subse- extraneous fragments, were identical quently identified as silicon carbide, within the error of measurement. In together with a small proportion of s An examination of the data found in the fol- "Refractive Index Measurements of Some Early lowing papers is the basis of this statement. American Glasses." Antiques 32 (6):310-312 Morey, G. W., and Merwin, H. E. "The Relation (1937). Between the Composition and the Density and 6 One method of measuring the transmission of I. "The Soda- the Optical Properties of Glass. would be the use of a Lange ocular photo- Lime-Silica Glasses." Jr. Opt. Soc. Amer. 22:632- light cell, together with the appropriate Jena glass 662 (1932). Wright, F. E. "The Manufacture of Optical Glass and Optical Systems." Ord. Dept., filters, in place of the regular microscope ocular. Doc. No. 2037 (1921). Geller, R. F., and Bunting, A description of the Lange ocular photocell ap- E. N. "The System K0-PbO-SiO;." Jr. Res., peared in the catalogue of Pfalz and Bauer, Inc., Natl. Bur. Stand. 17 (2) (1936). Faust, G. T. Empire State Bldg., New York City. CRIMINAL INVESTIGATIONS miscellaneous unidentified material. rather than the Grecian material. Un- The silicon carbide was identified by der the microscope, however, all three means of its characteristic optical prop- samples (the unknown, the Turkish erties: uniaxial positive figure,7 very emery, and the Grecian emery) were high index of refraction, partial hex- quite similar. In comparing the optical agonal outline of the broken grains, properties of the three specimens, it and the pleochroic8 character of the was found that the unknown resembled grains in various intensities of blue. the Turkish emery more closely than The grains varied in size from 0.24 x the other. The sand fraction consisted 0.18 mm. to 0.46 x 0.30 mm., or approxi- almost entirely of quartz, but an oc- mately a grain size of No. 60 and a U. casional grain of emery was observed. S. screen size about No. 40 (420 mi- On the basis of the petrographic ex- crons). The results of the micro- aminations it was possible to offer con- scopic study proved conclusively that clusive evidence which showed that the silicon carbide had been thrown be- composition of the broken wheel was tween the brake shoe and the cable. not the same as that of the products Another interesting case in which the of the manufacturer being sued. petrographic microscope offered a dis- Other Possible Applications tinct solution involved a broken grind- ing wheel. An injured workman al- The above illustrations represent leged that the emery wheel which broke only a few of the manifold possibilities and struck him was of a certain manu- offered by petrographic examination facturer's product. The wheel was of materials associated with criminal submitted for petrographic study, to- offenses. Further value for such ex- gether with samples of Turkish and amination is found in problems in which Grecian emery ores. A sample of the it is necessary to differentiate between unknown wheel was treated with a solid solutions, double salts, and mech- 0.5% sodium hydroxide solution to re- anical mixtures. Also, chemists or spec- move the bond. This sample was then trographers are unable to positively separated into several fractions on the identify polymorphic substances, such as basis of the respective specific gravi- for example, calcite and aragonite, which ties. The results of the analysis re- compounds are fairly common in in- 9 vealed emery (80%) and sand (20%). dustrial and natural products. How- The color of the questioned emery ever, identification of all such crystals viewed megascopically was black, and can be made by means of their optical in this respect the unknown resem- properties as determined by the petro- bled the sample of Turkish emery graphic microscope.
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