Two years ago, I noticed a touristy website that tried to pass off a Golden-mantled Ground Squirrel as being a Chipmunk (posting). Fascinated by this ineptitude, I searched stock-photo sites and discovered that it wasn’t uncommon to label pictures of a Golden-mantle Ground Squirrel as being a Chipmunk. As I noted at the time, one should not turn to stock-photo companies for biological insights.
The mistake is easy to make if the viewer applies an algorithm for squirrels: stripes mean chipmunk. The problem is that it isn’t true. While the chipmunk has multiple white stripes that extend from its back to its head (another example), the much larger Golden-mantled Ground Squirrel has two white stripes, one on each side, and neither extends to its head.
A Golden-mantled Ground Squirrel enjoys a snack high in a subalpine forest. The single white stripe on either side does not extend to the head.
The reason for the designaton of golden mantle isn’t always obvious, but here the golden pelage on the head and shoulders is abundantly clear.
Mach bands
Local wildfire smoke from this grim season had all but vanished when more flowed in from the south. As uncomfortable as it is, the smoke provides the hazy air that easily enables the identification of Mach bands.
Mach bands are not a feature of the external natural world. Rather, they arise in the eye and are an optical illusion first explained by Ernst Mach (1838–1916). The bands result from a process in our retinas that enhances contrast. Consequently, they appear subtly in everything we see, whether it is a view of the external world or just a picture of that view. However, the bands are never so apparent as when looking at step-like transitions in brightness. A succession of distant ridges seen through a smoky haze provides an ideal place to examine them.
Consider the receding ridges in the centre of this scene looking across the Lake. Each ridge seems to be edged with a thin diffuse dark band which contrasts with an adjacent brighter band on the ridge beyond it. Neither band is present in the external world; they are creations of our eyes.
Here is a detail from the centre left of the picture where one ridge passes behind another. The picture has been rendered in black and white, but the thin Mach bands are readily apparent.
When the brightness is plotted for the picture detail, the two cliffs marking the shift from one ridge to the next are apparent. Yet this quantitative analysis does not show the bands that the eye perceives: there is no small trench on the dark side of the cliff or small ridge on the bright side. Mach bands are an illusion created by one’s own image processing. Our subjective view differs from the objective scene.
