Monday 8th April marked a rare astronomical event across Western America - the first total solar eclipse since 2017. Despite living in England, where the solar eclipse was not visible, I still managed to view photographs taken by a friend who witnessed it at the time that captured the white halo of light hugging the dark silhouette of the passing moon. Although the beauty of the phenomenon was transmitted through the images, my friend spoke of another - somewhat supernatural - effect sparked by the eclipse that could not be captured by even the most high-resolution lens: a silvery-blue cast had descended over the scene, tainting the surroundings with its dusty tinge.
This effect is known as the Purkinje shift. To explain it, we must examine the anatomy of both the human eye, and the composition of the sunlight that reaches Earth.
The eye contains two types of specialised cells tasked with translating light input into an electrical output, a language understood by the brain that allows us to perceive the colour, shadow, and contour of the world around us. These cells are called rods, which detect light and dark, and cones, which pick up colour signals. Cones are able to detect the longer wavelengths of red and green, and the shorter, blue wavelengths; rods receive signals over a smaller wavelength range that encompasses the longer portion of blue light and short portions of red and green. However, the optimal wavelength for rod vision covers non of these zones - this is the reason that scotopic vision tends to be perceived in minimal colour.
The structure of the rods and cones are relatively similar - a region of isks which detect the ultraviolet light, followed by a mitochondrial cytoplasm zone and nucleus region linked by a cilial bridge, ending in a synaptic area that transfers the resulting signal. Ultraviolet light at a certain wavelength is detected by proteins called opsonins, which exhibit cis-trans isomerism upon the incidence of this light. These proteins exist alongside a side-chain of retinal which can absorb a photon, leading to the activation of transducin as the next step in the protein cascade. This is bound to guanosine diphosphate, which differs from adenosine diphosphate by only its nitrogenous base, This is changed into guanosine triphosphate, which causes the activation of a phosphodiesterase enzyme which hydrolyses cyclic guanosine monophosphate, lowering its overall concentration in the region of the cell. The falling concentration results in the closure of cation channels, so cations build up next to the membrane between the disks and cell body. This leads to the setup of a charge difference: an electrical signal, which passes over the terminal synapse through the optic nerve to the occipital lobe of the brain for further processing.
Photopic vision lies in the range of the cone wavelengths, and these are the cells which are thus activated more during the day. However, during an eclipse, a new type of vision distinct from either scotopic at night and photopic in the day is employed: mesopic vision. This vision range lies between scotopic and photopic and therefore uses both rod and cone output simultaneously. As covered earlier, the rod range falls over blue, red and green areas, yet predominantly lies overlapping with the blue wavelength field. Thus during an eclipse, when both rods and cones are being fired at the same time, the rod output tends to align with the blue wavelength output and the main signals received are towards the blue end of the spectrum: the Purkinje shift explained biologically.
Additionally, this phenomenon can be explained using the physics of the light that reaches our planet from the sun. Direct sunlight composes all visible (and some invisible) wavelengths of light and these create the white light of daytime. The explanation for the sky's blue wash is simple: blue wavelengths are shorter and therefore more easily scattered by the randomly moving air molecules. It is this scattered light which reaches our eyes as a reflection, as red and green light is more easily transmitted to make sunlit objects look more yellow, rather like a child's drawing of a bright, dandelion sun in the corner of a blue page. On the other hand, during an eclipse, we do not receive direct light - instead all light that passes to us reaches the Earth indirectly as the direct light has been shielded by the moon. And this indirect light - the same as the blue of the sky - consists of the most easily scattered blue wavelengths. With a reduction in the amount of red and green light reaching the ground, my friend noticed a slight blue shroud to the landscape: a physical product of our unique anatomy and the diversity of spectrum wavelength that can only be admired in person, with no records existing as no camera has yet been able to perfectly replicate the intricacies (and natural flaws - wavelength 'blind spots') in our rod and cone cells that allow this change to be detected.