This is a complete set of British Ogden Cigarettes "Optical Illusion" cards, included as freebies inside cigarette packs, and which someone very diligently collected to have all 25 that were issued. I think the graphics on these are just fantastic, and might argue they they are the best looking set Ogden's ever produced, and they produced a lot, beginning in the late 19th century. 

On the back of each card is an explanation of each illusion, or in some cases instructions for how to create an illusion according to the diagram on the face of the card, which in a few cases seems a fairly cumbersome endeavor! These came in plastic sleeves with slots that fit them exactly, and I will ship them protected in those, though I'd be most inclined to arranged these in a grid, frame them up, and zone out staring at them!

Each card measures 2 11/16" x 1 7/16". The 25th card shows more wear than the others; overall these are in very good shape, for the most part very bright and clean and crisp.