"In <em>optics</em>, a deviation in the rays of light when unequally refracted by a lens or reflected by a mirror, so that they do not converge and meet in a point or focus, but separate, forming an indistinct image of the object, or an indistinct image with prismatically colored edges."-Wright, 1902

Aberration

"In optics, a deviation in the rays of light when unequally refracted by a lens or reflected…

An illustration depicting an astigmatism. An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.

Diagram Illustrating Astigmatism

An illustration depicting an astigmatism. An optical system with astigmatism is one where rays that…

"An optical instrument employed to facilitate the sketching of objects from nature by producing a reflected picture of them upon paper. Wollaston's apparatus is one of the commonest. The essential part is a totally-reflecting prism with four angles, one of which is 90 degrees, the opposite one 135 degrees, and the other two each 67 degrees. One of the two faces which contain the right angle is turned towards the object to be sketched." &mdash; Winston's Encyclopedia, 1919

Camera Lucida

"An optical instrument employed to facilitate the sketching of objects from nature by producing a reflected…

"An optical instrument employed for exhibiting the images of objects in their forms and colors, so that they may be traced and a picture drawn, or may be represented by photography. A simple camera obscura is presented by a darkened chamber into which no light is permitted to enter excepting by a small hole in the window-shutter. A picture of the objects opposite the hole will then be seen on the wall or on a white screen placed opposite the opening." &mdash; Winston's Encyclopedia, 1919

Camera Obscura

"An optical instrument employed for exhibiting the images of objects in their forms and colors, so that…

"The optical lantern is an instrument for projecting on a screen magnified images of transparent photographs, paintings, drawings, etc." &mdash; Avery, 1895

Optical lantern

"The optical lantern is an instrument for projecting on a screen magnified images of transparent photographs,…

"Diagram showing how the lens changes its form for near and far sight." &mdash; Tracy, 1888

Eye lens

"Diagram showing how the lens changes its form for near and far sight." — Tracy, 1888

An illustration of a petrographical microscope. A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks.

Petrographical Microscope

An illustration of a petrographical microscope. A petrographic microscope is a type of optical microscope…

"An instrument described by Foucault for transmitting a beam of light along the optical axis of a fixed horizontal telescope."&mdash;Finley, 1917

Siderostat

"An instrument described by Foucault for transmitting a beam of light along the optical axis of a fixed…

The Z&ouml;llner illusion is a classic optical illusion named after its discoverer, German astrophysicist Johann Karl Friedrich Z&ouml;llner. In this figure 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 may be that the Z&ouml;llner illusion is caused by this impression of depth.

Zollner Illusion

The Zöllner illusion is a classic optical illusion named after its discoverer, German astrophysicist…