How to see Stereoscopically
When you see two similar photographs mounted side by side on a card, you likely are looking at a stereoview. The photographs of a stereoview are taken at a small distance separation in order to fuse into a three-dimensional image. They are presented throughout this digital book in a size to facilitate stereo-viewing similar to that which the Victorians experienced. If you are unfamiliar with viewing in stereo (3D), patience is the key. You may see in stereo at the first try, or it may take several attempts.
Free viewing
To see the stereoviews in 3D without a device, square your head in front of the pair of images. Then, cross your eyes and hold them in that position. This will not hurt your eyes. You will see two blurry images, then a central image, which will fuse into 3D. If this is not successful, you may need to move a little closer or further away from the screen. Once the central image fuses into 3D, stay at that position. The longer you hold the image, the stronger will be the perception of depth.
Viewing with a lorgnette stereoscope
Some people prefer to use a lorgnette viewer to see stereoviews on the computer in 3D. Here's how to use one: place the viewer toward the end of your nose and draw your head toward the pair of images. You will notice two images at first. These should quickly resolve into a clear, single view in the center. The longer you view, the more you will perceive depth.
Links to purchase
http://www.3dstereo.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=SVN-DLOR&Category_Code=
The Loreo lite 3D viewer
http://www.loreo.com/pages/products/loreo_lite_3d_viewer.html
Owl viewers, The London Stereoscopic Co.
http://shop.londonstereo.com/lsc-owl-viewer.html
The Twin Scope Viewer
https://www.colleenwoolpert.com/TwinScope-Viewer