That is a Viewmaster. Those things are AWESOME.
It is indeed a viewmaster. It would probably be considered a toy. It came with or you bought extra sets of slides (small transparent pictures) along the outer edge of a circular cardboard disk, about the size of a CD. There were 2 slides for each picture, and it gave you a 3D view of the item pictures. Theres a lever on the side that you pulled and it would index the disk so you could look at another picture.
Yep view-master. They were really cool. They worked by placing a circular slide strip in, which contained about eight different pictures, and they were really cool 3D looking. When I was young I actually collected all the super hero slide, ie batman, super man wonder woman....
Below is a link to learn more about them:
EDIT: sorry. john already posted the link while I was formulating my answer.
it looks like a view finder
It is a viewmaster.