View-Master is the trademark name of a line of special-format stereoscopes and corresponding View-Master "reels", which are thin cardboard disks containing seven stereoscopic 3-D pairs of small color photographs on film.
The View-Master system was introduced in 1939, four years after the advent of Kodachrome color film made the use of small high-quality photographic color images practical. Tourist attraction and travel views predominated in View-Master's early lists of available reels, most of which were meant to be interesting to users of all ages. Most current View-Master reels are intended for children.
Source: Wikipedia, "View-Master", available under the CC-BY-SA License.