a diverging lens mounted at the bottom of a tube the eyepiece fits down into, and effectively cuts the focal length of any eyepiece

a nice addition as well, doubling the magnification of your present eyepieces

a specially designed lens that increases the magnification of any eyepiece it is used with

A folding knife design that is more than 150 years old. This was an inexpensive knife, always a one or two blade jackknife pattern with longer than normal bolsters, sought after by collectors.