The Stereoscopic cards in this collection date from late 1800s to early 1900s. Stereoscopic Cards were very popular as they created a 3D view of the image transporting the user into the world of the image.
Using a special card holder the user would view the image by sliding the card holder from the view window either closer or further from the eyes to adjust focus of the image. There are 3 of these stereoscopic viewers in the Following The Lights Collection, see the images below in the gallery: SCV001, SCV002, SCV003.
There have been modern versions of viewers through the 1900s including digital 3D glasses that are used today for watching movies rather than a still image. More information about postcard history can be found here: https://en.wikipedia.org/wiki/Postcard
This Stereoscopic image taken in 1908 from the deck of the NLV Hesperus or NLV Pharos on the tour by Northern Lighthouse Board Commissioners.
Viewers in the collection include SCV001, SCV002, SCV003.