The Necker cube is an optical illusion first published as a rhomboid in 1832 by Swiss crystallographer Louis Albert Necker. It is a simple wire-frame drawing of a cube with no visual cues as to its orientation, so it can be interpreted to have either the lower-left or the upper-right square as its front side; the Necker cube is an ambiguous drawing. The effect is interesting because each part of the picture is ambiguous by itself, yet the human visual system picks an interpretation of each part that makes the whole consistent; the Necker cube is sometimes used to test computer models of the human visual system to see whether they can arrive at consistent interpretations of the image the same way humans do. Humans do not see an inconsistent interpretation of the cube. A cube whose edges cross in an inconsistent way is an example of an impossible object an impossible cube. With the cube on the left, most people see the lower-left face as being in front most of the time; this is because people view objects from above, with the top side visible, far more than from below, with the bottom visible, so the brain "prefers" the interpretation that the cube is viewed from above.

Another reason behind this may be due to the brain's natural preference of viewing things from left to right, therefore seeing the leftmost square as being in front. There is evidence that by focusing on different parts of the figure, one can force a more stable perception of the cube; the intersection of the two faces that are parallel to the observer forms a rectangle, the lines that converge on the square form a "y-junction" at the two diagonally opposite sides. If an observer focuses on the upper "y-junction" the lower left face will appear to be in front; the upper right face will appear to be in front. Blinking while being on the second perception will cause you to switch to the first one, it is possible to cause the switch to occur by focusing on different parts of the cube. If one sees the first interpretation on the right it is possible to cause a switch to the second by focusing on the base of the cube until the switch occurs to the second interpretation. If one is viewing the second interpretation, focusing on the left side of the cube may cause a switch to the first.

The Necker cube has shed light on the human visual system. The phenomenon has served as evidence of the human brain being a neural network with two distinct possible interchangeable stable states. Sidney Bradford, blind from the age of ten months but regaining his sight following an operation at age 52, did not perceive the ambiguity that normal-sighted observers do, but rather perceived only a flat image. During the 1970s, undergraduates in the Psychology Department of City University, were provided with assignments to measure their Introversion-Extroversion orientations by the time it took for them to switch between the Front and Back perceptions of the Necker Cube; the orientation of the Necker cube can be altered by shifting the observer's point of view. When seen from apparent above, one face tends to be seen closer; the Necker cube is discussed to such extent in Robert J. Sawyer's 1998 science fiction novel Factoring Humanity that "Necker" becomes a verb, meaning to impel one's brain to switch from one perspective or perception to another.

The Necker cube is used to illustrate how vampires in Peter Watts' science fiction novels Blindsight and Echopraxia have superior pattern recognition skills. One of the pieces of evidence is that vampires can see both interpretations of the Necker Cube which sets them apart from baseline humanity. Multistable perception Rhombille tiling Spinning Dancer Ambigram History of the cube and a Java applet

Drizzt Do'Urden's Guide to the Underdark is an accessory for the 2nd edition of the Advanced Dungeons & Dragons fantasy role-playing game, published in 1999. Drizzt Do'Urden's Guide to the Underdark details the Underdark in the north and west of Faerûn in the Forgotten Realms setting, including the city of Menzoberranzan, the book has Drizzt Do'Urden as its nominal guide; the guide starts with an introduction that defines the physical boundaries of the Underdark, describes the intent and organization of the book and gives a brief list of D&D materials which have a strong connection to the Underdark. Drizzt Do'Urden's Guide to the Underdark was published by Wizards of the Coast, written by Eric L. Boyd. Drizzt Do'Urden's Guide to the Underdark was reviewed by the online version of Pyramid on January 21, 2000; the reviewer contends that this book continues the tradition of dungeon exploration in fantasy role-playing games to campaign worlds with dungeons, stating that with this book "we move to the next step -- campaign worlds that are dungeons".

The reviewer notes that the Underdark is "one of the most well known facets of the Forgotten Realms", with Menzoberranzan and "its most famous denizen", Drizzt Do'Urden being "some of the most famous pieces of the Realms". Shannon Appelcline wrote that it "is the single-most comprehensive sourcebook on the realms of the Underdark that lie beneath the Sword Coast, it talks about the major peoples of the Underworld and details dozens of cities, including Menzoberranzan" and that "Boyd's extensive research results in Underdark being full of tiny references. For example, drow cities are drawn from the list in Drow of the Underdark and from obscure references in Menzoberranzan"

Old Hickory Lake is a reservoir in north central Tennessee. It is formed by the Old Hickory Lock and Dam, located on the Cumberland River at mile 216.2 in Sumner and Davidson Counties 25 miles upstream from Nashville. The city of Hendersonville is situated on the northern shoreline of the lake, Old Hickory, a portion of Metropolitan Nashville-Davidson County, is located on the southern side of the lake, just upstream of the lock and dam; the lake extends 97.3 miles upstream near Carthage, Tennessee. The dam and lake are named after President Andrew Jackson, who lived in the vicinity, at The Hermitage; the lock, dam and lake are operated and supervised by the U. S. Army Corps of Engineers' personnel under the direction of the District Engineer at Nashville. Construction started in January 1952, dam closure was completed in June 1954. Historic Rock Castle, former home of Daniel Smith, known for his contributions in settling Hendersonville, TN, is located along this lake. Old Hickory Lake is a mainstream storage impoundment on the Cumberland River operated by the U.

S. Army Corps of Engineers; the reservoir extends 97.3 river miles. Water level fluctuations are minimal with minimum pool elevation at 442 feet. Public facilities include eight marinas, two Corps-operated campgrounds, 41 boat access sites, as well as the Old Hickory Lake Arboretum; this article incorporates public domain material from the United States Army Corps of Engineers website http://www.lrn.usace.army.mil/op/OLD/rec/project.htm. House of Cash Old Hickory Lake - official site