Stereo pairs do not require special viewers, but do require you to refocus your eyes (see instructions below).
To view the 3D effect of stereo pairs, the images must be lined up next to each other as pictured above. You need to cross your eyes so that the two images converge and become overlaid on top of each other. The white dots are provided to help you get started — you might like to practice with the dots first and then move to the picture.
As your eyes get used to the new position you should see a new image form in the middle as illustrated below. The images on either side are duplicate images which you learn to ignore.
This technique can take quite a bit of practice and getting used to. Despite the awkwardness of viewing and limitations on the image size, this method does provide excellent quality images because the original colours are fully retained.