During three trips to London at the turn of the 20th century, Claude Monet painted more than 40 versions of a single scene: the Waterloo Bridge over the Thames River. Monet’s main subject was not the bridge itself, however; he was most captivated by the landscape and atmosphere of the scene, with its transitory light, fog, and mist.

Eight paintings from this series of London fogs are the centerpiece of the Memorial Art Gallery’s exhibition “Monet’s Waterloo Bridge: Vision and Process.” A recognized master of landscape painting, Monet was an integral founder of the Impressionist movement, which embraced the philosophy of expressing the fleeting sensory effects in a scene.

But how does Monet depict the same scene at different times of day and in various conditions? And how does a viewer see an artist’s brushstrokes of color as a cohesive image, and vastly different colors as the same bridge?

With each of the paintings in the series, Monet manipulates viewer perception in a way that scientists at the time did not completely understand. Today, research such as that conducted at the University of Rochester’s Center for Visual Science, founded in 1963, provides insight into the complexity of the visual system, illuminating Monet’s processes and the intricacies of his work.

The Memorial Art Gallery partnered with the Carnegie Museum of Art and the Worcester Art Museum to analyze the pigments of color Monet used in his Waterloo Bridges series. They found that Monet used a very limited color palette in his Waterloo Bridge series, but was still able to evoke a wide range of ambiances. How did he do this?

The answer involves how our eyes take in wavelengths of light, which our brains interpret, says David Williams, professor of optics at Rochester and the director of Rochester’s Center for Visual Science. In the retina of the eye, there are three types of cones: blue, which is sensitive to short wavelengths of light; green, which is medium-wavelength sensitive; and red, which is long-wavelength sensitive. Each type of cone either reflects or absorbs the various wavelengths of light. These trichromatic signals “are very simple, yet the million shades of color that we experience are derived from just those three,” says Williams, who, in the 1990s, was the first person to image all three kinds of cones in a living human retina and identify how the cones are arranged.

From the retina, signals travel along the optic nerve to the visual cortex, one of the most primitive areas in the back of the brain. Signals are then transmitted back and forth between the visual cortex and other higher-level parts of the brain, including those involved in attention, memory, experience, and biases. The brain’s job is to integrate sensory information from the eyes into pieces—lines, shapes, and depth—and construct them into objects and scenes.

The illumination of an object, for example, can alter perception. That’s because what arrives at our eyes when viewing an object is a combination of both the illumination falling on the object and the intrinsic properties of the object itself, Williams says. “Your brain has a real challenge, which is to figure out what is true about this object even though what arrives at your eye is radically different depending on how it is illuminated.”

When you take an object like a white sheet of paper, it will always be intrinsically white—a phenomenon known as color constancy. If you put the paper outside, it will still be white in the morning light, in the middle of the day, and when the sun goes down, although “if we were to make objective measurements of the light entering your eye in those various circumstances, they would be very different,” he says.

The Waterloo Bridge itself never changes color, but Monet paints it using color in different luminance (brightness), value (a color’s relative lightness or darkness), and intensity (a color’s saturation) to depict sunrise, direct sunlight, and dusk. The brain is able to take in the illumination washing over the entire scene, integrate information, and make inferences. If objects have a bluish cast, for instance, the brain is able to infer that it is most likely daytime. If objects have a reddish cast, the brain infers that sunset is most likely approaching, Williams says. Ultimately, “Monet was highlighting how different an object can be, depending on how it is illuminated. But any normally sited person looking at this series will know: the bridge is gray brick, no matter what time of day it is.”

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