Black and white have an intimate relationship.
Together they carry the connotations of good and evil, light and dark, positive and negative, yin and yang. Without one you cannot have the other.
Traditionally, black ink has been printed on white paper. As goes so many technological advances, cost is involved. So much so, that ink was expensive and minimal usage was necessary. However, other understandings emerged from this circumstance. Specifically, the knowledge that ink/printing on paper is a reflective technology. Meaning, that (sun) light bounces off of paper to create an image on the eye.
With modern technology, such as television and computer displays, they are transmissive. Thus, transmitting light to create an image on the eye.
While the eye is attracted to an object in a blank space, such as large white circle on a black printed page, what also should be considered is what is easiest or more comfortable on the eye. The near equality of use of black on white with both paper and computer displays is the focus here.
As a transmissive technology, computer displays beam large amounts of artificial light when its display is white. A large portion of the Internet's web sites have a white background. My guess would be around 80%.
This seems to me that people (and even designers) believe the print world and digital world to be equivalent to present information. Obviously, that is not true. However, omitting the additional traits that digital information can be dealt with, let us remain with the colors on a static presentation.
With completely white screen on a computer display, all pixels are on. Thus, a bombardment of transmissive light enters the eye. This is counter-intuitive to a natural eye attraction to an object.
Going back to the circle presented, we can interchange the focus appropriately for the medium (paper or computer display). A white circle on a black background for a computer display would focus all of the emitted light from the circle, which would naturally attract the eye there (both from an energetic/active level as well as an object to view). A black circle on white paper brings the eye to it because it has modified the paper by adding ink to it.
In summary, best usage of:
print = black on white
screen = white on black
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