The Daily Omnivore

A hopper crystal is a form of crystal, defined by its ‘hoppered’ shape: the edges are fully developed, but the interior spaces are not filled in due to rapid growth. This results in what appears to be a hollowed out step lattice formation; the interior edges still show the crystal form characteristic to the specific mineral, and so appear to be a series of smaller and smaller stepped down miniature versions of the original crystal.

Hoppering occurs when electrical attraction is higher along the edges of the crystal, causing faster growth at edges than near face centers. This attraction draws the mineral molecules more strongly than the interior sections of the crystal, thus the edges develop more quickly. Hoppering is common in many minerals, including lab-grown bismuth, galena, quartz (called skeletal or fenster crystals), gold, calcite, halite (salt), and water (ice).

An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in one’s vision after the exposure to the original image has ceased. One of the most common afterimages is the bright glow that seems to float before one’s eyes after looking into a light source for a few seconds. The phenomenon of afterimages may be closely related to persistence of vision, which allows a rapid series of pictures to portray motion, which is the basis of animation and cinema.

Closing the eye can help achieve a better sense of the color in its own aspect. Afterimages come in two forms, negative (inverted) and positive (retaining original color). The process behind positive afterimages is unknown, though thought to be related to neural adaptation. On the other hand, negative afterimages are a retinal phenomenon and are well understood.

Illusory contours or subjective contours are a form of visual illusion where contours are perceived without a luminance or color change across the contour. A classic example of illusory contours is the Kanizsa triangle. This figure comprises three black circles with equal wedges cut out of them facing the center point and three black angles on a white background.

But many observers see a white triangle on top of three black disks and an outline triangle. The white triangle appears brighter than the white background and shows a contour even in regions where there is no luminance change in the image. The Ehrenstein illusion is another common form of illusory contours. It is thought that early visual cortical regions such as V2 are responsible for forming illusory contours.

The Zöllner illusion is a classic optical illusion named after its discoverer, German astrophysicist Johann Karl Friedrich Zöllner. In 1860, Zöllner sent his discovery in a letter to physicist and scholar Johann Christian Poggendorff, who subsequently discovered the related Poggendorff illusion, in the original drawing by Zöllner. In the illusion the black lines seem to be unparallel, but in reality they are parallel. The shorter lines are on an angle to the longer lines. This angle helps to create the impression that one end of the longer lines is nearer to us than the other end.

This is very similar to the way the Wundt illusion appears. It is also is similar to the Hering illusion and the Müller-Lyer illusion. All these illusions demonstrate how lines can seem to be distorted by their background.