At the lens end of the instrument and back of the carriage is a diffusing screen of opal glass illuminated by an ordinary 75-watt light bulb in a desk type reflector, thus providing a white background for the objects being photographed.
There are two adjustable, microscope-type object illuminators (Spencer) which are so positioned that they give oblique lighting to the specimens. These lights are controlled by two rheostats (Fig. 65) so that their intensity can be matched or balanced. This is important, as a test bullet is usually brighter than the evidence bullet.
Another similar pair of lights (not shown in the illustrations) is also used on the opposite -side on occasion, or both sets can be used, to get the clarity and definition that are so necessary to good photography. Figure 66 shows a typical example of what can be accomplished with a pair of bullets that show good matching. Below the horizontal dividing line is shown the base end of the evidence bullet, above the line is shown the nose end of a test bullet fired from the suspected gun.* For other examples see Figs. 67 to 74.
*This instrument and a number of others were built in collaboration with the author by Mr. Lee K. Henke, Senior Mechanician in the Shop of the Department of Chemistry at the University of Wisconsin.
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