JAN PURKINJE, A 19TH-CENTURY CZECH physiologist known for recognizing that fingerprints can be useful for identification, also discovered an odd thing about human vision. As light dims at day's end, colors start to look gray, but blue hues linger well after other colors have faded. In bright light, the eye is most sensitive to yellow-green light. In near darkness, Purkinje found, red hues vanish, and the peak sensitivity shifts to green blue, a change now known as the Purkinje shift.
These quirks arise from the two types of receptors built into the human eye. Cone-shaped cells yield sharp color vision but work only in bright light. Rod-shaped cells are far more sensitive to dim light, but they do not pick up color and cannot detect red light. That's why natural landscapes appear gray in dim moonlight but blue green by the full moon. Go outside when the moon is full in mid-April and you'll see that distant red flowers (able to rouse only the rods) look gray, while nearby grass (bright enough to tickle the cones) still appears green.
The implications of all this for astronomy are sobering. Galaxies, nebulas, and almost all other deep-space objects appear dim even through large telescopes, so at best we see only a portion of their true colors. For the dimmest objects, we see no color at all.
Stargazers rely on various tricks to circumvent these limitations. They look off to the side when examining faint galaxies or trying to spot dim stars. This technique—called averted vision—causes celestial features suddenly to pop into sight. The reason: The eye's center of vision is packed only with cones. A sideways glance shifts the light onto the much more sensitive rods in the periphery of the retina. Astronomers also let their eyes "dark adapt" for about 15 minutes before observing, because rods take a long time to reach full sensitivity. And astronomers illuminate their sky charts with red light, which makes objects visible to the cone cells without affecting the red-blind rod cells and forcing the dark-adaptation process to begin all over again.
All the cunning in the world can't bring dim colors into view, however. Take the famous Orion nebula, now in the southwest at nightfall. In photographs, this gassy cathedral of star birth is full of lyrical blue-and-red sweeps of dusty gas. But to the eye, it appears a drab gray through small instruments, a slightly less drab green through larger ones.
The reason is that the Orion nebula's dim glow permits color perception only in the middle of the spectrum. Human vision can sense the nebula's relatively bright, greenish emissions from ionized oxygen, but nothing else. Photography picks up fainter, outlying blues from light scattering off dust particles and reds emitted by hydrogen.
If astronauts ever travel to the Orion nebula, they'll anticlimactically observe much the same colors they had seen from their own backyards. The nebula's light is just too feeble and spread out. Similarly, no space voyage will ever allow humans to observe the dramatic blue spiral arms and golden cores of galaxies seen in astronomy posters.
The marvelous thing is that we know about those colors at all. Only through photographic film and digital detectors can we transcend the Purkinje shift and perceive the full rainbow richness of the cosmos.