I recommend the OWL stereo card viewer from the London Stereoscopic Company. Viewing this site on a iPad should work with the viewers from this site. The card images are designed so that they fit inside the aperture at the back of the image. To avoid scratching your iPad screen, you can apply small silicon pads to the back of the viewer, which should help reduce slippage. Using any viewer that comes into contact with your iPad is done at your own risk. It is not my fault if you scratch your screen.