next, investigators asked dick bisbing, senior research microscopist at mccrone associates, to analyze the dirt on the bottom of larry bruce's shoes, which were still in evidence. to do this, bisbing used a polarized light microscope. >> it's like looking through polarized sunglasses. and by manipulating the light, we can see different optical properties, different optical features of each of these mineral grains. their appearance, their color, their optical properties all help us identify the type of mineral. >> the dirt on larry's shoes did not match the samples of dirt around judy's body. but the polarized light revealed an important, previously undetected clue -- these brightly colored minerals. >> the soil from the shoes contained calcite, which is calcium carbonate, could be from limestone, and that type of material is often used in roadways, road beds. >> near judy's body was a roadway that had been paved with new limestone gravel just days before her body was dumped there. this was the first potential link between larry bruce and where judy's body was found. next, investigators