The autostereogram

 

 

Is a way to look at 3D images without any help.

 

There are two ways to do it. One is to kinda try to look (focus your eyes) about eight inches behind the picture until you can see the 3D image.

The other is to cross your eyes and slowly uncross them until you see the picture.

 

This one- when you finally get it- is Legionnaires standing in columns.

From Here  to see an enlarged ( and easier to see) pic.