The telephone rang at 11 p.m. It was our daughter calling and her message was urgent, “Come outside now!”
Despite the near freezing temperature, we, as dutiful parents, heeded our child’s cry. Apparently she had something to show us and were to meet her at the old farm gate.
Flashlights jerkily illuminated pine trees and garden fences as we scuttled over to her position. “Look up!” she exclaimed upon our rendezvous.
Stars. OK. Clouds. OK. So just what is it we are supposed to be see ... hey, that cloud just shot across the entire sky! The longer we looked, the more bizarre the skyscape became. Oh, I get it ... northern lights!
I must admit, I was under-impressed with the lack of colours (although her cell phone pics revealed the pinks and greens with pretty good clarity). We stood huddled in the cold for almost an hour, witnessing the most awesome show of what can best be described as wraiths bursting forth from the northern horizon. A real Harry Potter moment.
And then another amazing scientific event happened ... we could see in the dark! Individual branches could be discerned from the nearby black blob of a pine tree; we could easily locate each other without flicking on a flashlight. Our night vision had kicked in!
Night vision is very different from day vision. During the sunlit hours, our eyes see colours everywhere, and the messages sent to our brain are diagnosed, studied, and file-matched with previously seen things, thus providing us with the correct answer: I am looking at a ginger cat on a green lawn with red coneflowers in the background.
But at night those reflective colours are not available and our eyes must compensate to keep the brain happy with incoming information. If colour is not available, at least send shape files!
At the back inside of our eyes are structures called cones which react to the different wave lengths of light coming in. Depending on the strength of a certain wavelength, the cone will interpret that pixel as red, yellow, blue or a combination thereof.
A whole bunch of cones sending a simultaneous message to the brain provides the raw data for an interpretation of whatever it is we are looking at.
But it’s not just colour data that’s coming from our eyes, there are also shapes and textures to be analyzed. This is done by the rods that surround our retina, again blinking on or off to capture the dimensions of a shape that will mix with the colour scheme that is being concurrently transmitted.
However, when no colour input is happening (because it’s dark outside), the brain has a bit of a panic attack and demands more data input from something else. This is where the rods step up their usefulness.
Without the distracting colour data flowing in, the shape files from the rods becomes the dominant form of brain input. And the longer the eyes are allowed to adapt to the lack of colour, the greater the clarity of the image. This process is known as ‘night vision.’
Since the rods are around the periphery of the retina they can pick up information that would normally be considered as being “out of the corner of your eye.” And without the noise of colour interpretation (which comes straight in to hit the cones at the back of the eye), the brain becomes hyper aware of shapes and movements to your side.
All of which explains why we jump with surprise when walking through dark places ... we are not used to relying of the subtle movements of things off to the side.
As the overhead light show seemed to be coming to the end of its performance, flashlights were clicked on to ensure we all made it safely back to the house without experiencing a trip hazard. But I was looking for something else that may be caught in the sweep of a flashlight: eye shine!
Eye shine is the reflection of the back of an animal’s eye when it looks at you from the confines of the protective dark. Night animals, such as deer or rabbits, have rather large eyes with lots of rods and only a few cones, as colour detection at night is not nearly as important as noticing the shadowy movement of a potential predator.
If a beam of light (car headlight or flashlight) hits the animal head-on, a reflective colour can be seem (by the panicked driver or ‘just peed my pants’ camper). There is a layer at the back of their eyes called a tapetum lucidum (which is Latin for ‘bright tapestry’) and depending on the species, a certain colour is noted when illuminated by a light source.
Rabbits tend to have a bright red eye shine, while deer reflect white. Owls tend to show red or orange. Green eyes? You are looking at a fox, dog or bullfrog. Raccoons and bears tend to glow yellow.
These colours can shift a bit from one animal to another even if they are the same species. Domestic cats should glow green, but the amount of zinc or riboflavin in their diet will shift the reflective ability of their tapetum lucidum to show red or orange.
Interestingly, humans do not have a tapetum lucidum, so the best we can hope for is ‘red eye’ in a photo, which is a recording of all the cones screaming “arrrgh” at being hit simultaneously by a very bright light (camera flash).
By the time we arrived back to the shelter of our warm house, we had not one sighting of a foraging white-eyed skunk or skulking red-eyed rabbit. Whew. Nighttime .... it’s a whole other world of discovery.