This illusion shows the interaction between color perception and afterimages. First, notice that the left image has a color imbalance to the right and left of the fly. Now, fixate your gaze on the fly in the right image for 10 to 30 seconds: this staring will selectively adapt the blue versus yellow color channels of your retina in your left versus right visual field. As a result, your left visual field will become less sensitive to blue and your right visual field less sensitive to yellow. Then look back at the fly on the cow’s nose, and the image will appear to have a perfect color balance. This illusion helps to explain why objects look the same color under different lighting environments. For example, your shirt looks the same under incandescent lighting and under sunlight (these two light sources have different color spectra): part of the system that produces color constancy in our perception also plays a role in this slide’s illusion. Notice, too, that the color-selective adaptation is still constrained to a single eye: if you close one eye during the adaptation period and then switch eyes while looking at the cow, the color balance will revert to blue/yellow in the unadapted eye.