A Perpendicular Shadow

I nudged past the brass cogwheel... slowly, keeping low. Just beyond the wooden seed tray, someone had left an upright rectangular glass microscope slide propped against an empty clay saucer. Was it going to fall? My breath hitched. I crept forward on both knees, testing the glass edge with one gloved knuckle. Firm friction. The glass threw a thin, razor-straight rectangle of noon light across the cedar grain. Holding my green die tight between both thumbs, I slid it along the wood until its edge caught the precise perimeter of the bright patch. Click. The cube nested exactly inside the lighted corner, flush against the shadow line. Nothing shifted. My chest finally stopped fluttering.