What are stereo pictures?
  Everyone is familiar with the appearance of a two dimensional picture. What is generally not understood is that a three dimensional stereo picture is not looked at but experienced.
3-D pictures were immensely popular throughout the world from about 1890 to 1920. Most homes had a 3-D viewer and a collection of 3-D pictures showing scenes from around the world. The family would gather and look at the pictures as modern families gather to watch television in the present time.
How you experience a 3-D photograph.
When you look at anything (not a picture) each of your eyes sees a slightly different image of the object because they are seperated by a distance of about 2.25 inches. Your brain takes these two different images and constructs a 3rd image which is what gives you the three dimensional view of real life.
  
 

If you have never seen stereo pictures you have a real thrill ahead of you. If you look at a two dimensianal photograph of the Grand Canyon you are merely looking at a picture. If you look at a three dimensional photograph of the Grand Canyon you feel like you are standing there on the rim, looking down.

When you look at a two dimensional picture each of your eyes sees exactly the same image so your brain can only see that 2 dimensional image. But with stereo photography, the camera takes two seperate pictures with two seperate lenses exactly the same as you would see with your two eyes. When these two different images are viewed through your eyes, your brain constructs that magical third image that is exactly the same as it would have done if you were standing at the position of the stereo camera when the images were made.
 

 

Morikami Japanese Gardens and Museum Building by Barry Stier

Above is a two dimensional picture with a great feeling of depth.

To the left is the same scene photographed as a three dimensial STIEREO view. 

Each of the two views to the left are actually slightly different.

 
If you have a stereo viewer or if you know how to "free view" a stereo pair you can see this same picture clearly in actual  three dimensions. You would feel like you could walk right into the garden.
 
     Stereo photographs and stereo viewers first became popular around 1890. It was a popular form of home entertainment at that time. You can still
buy those old time viewers in antique stores and they still work with modern stereo pictures.