"Retinal disparity" refers to visual perception that creates dimensionality and depth due to viewing the same object from two slightly different vantage points because of two eyes. This term is also known as binocular disparity. This type of vision lets animals, including humans, use depth perception as a way to interpret visual stimuli.
The human brain combines an image from the left and right eye to create three-dimensional perception. The difference between the two images is known as disparity. The closer an object is to the eyes, the larger the disparity between two retinal images.
One way to see this disparity is to hold a pencil with a vertical clip on it 18 inches from the nose so the clip is just visible to the left eye. Then close the left eye and open the right eye to see the clip is not present from the disparate view. Beyond 30 feet, the difference between images is small enough to be inconsequential.
Stereoscopic vision using hidden images creates an enhanced three-dimensional effect. Relaxing the eyes fuses two special images together to form a new image. Creating the right amount of retinal disparity brings the hidden visual to the forefront. These types of images were popular in calendars, posters and newspapers.