In addition the SIRDS are used in psychological tests and memory tests in some areas of visual psychology. Perhaps the most popular of these puzzle books was the Magic Eye books of the nineteen nineties. While the initial use was for research and information, which is still being carried out, SIRDS soon found a following of commercially available "puzzles" and eye games purely for fun. This allows for research and tests to be set up and performed to measure the perception of the brain and how it communicates to other portions for depiction and understanding. The concept is that each eye can see a different image and that the brain reconstructs the two images into a single frame. The main use of SIRDS is in the understanding of how the brain perceives light, motion, three dimensional depth perception and how the eyes communicate to the brain via nerves. The individuals working at Bell labs noticed the effect when scanning photos of camouflaged aerial photographs of military installations and outposts. In the nineteen fifties work at Bell Laboratories led to the discovery that depth perception is not a function of the eyes but rather interpreted by complex neurological mechanisms in the brain. He separated an image into two different mirrored images and placed them on an apparatus to demonstrate that the eyes can perceive three dimensions from two dimensional images. The original stereoscope was developed by Charles Wheatstone in 1838 in an effort to understand the mechanisms of depth perception in humans.