Flippers that have the ability to adjust to your pupillary distance! Small or big - not a problem you can easily move the lens! Flippers have a multi-faceted usage in the ophthalmic industry. Use flippers to measure the over-refraction in contact lenses and glasses. Use flippers to test and train the following conditions - accommodative facility, accommodative excess and accommodative insufficiency. Flippers are not only useful in accommodative testing and training but also for convergence and divergence. THINK BIM BOP - the minus side of the flippers acts like a BASE IN PRISM (equivalent to divergence training) and the positive side of the flippers acts like BASE OUT PRISM (equivalent to convergence training).