These eyepieces work well with the very long focal length telescopes (in Huygens day they were used with single element long focal length non-achromatic refracting telescopes, including very long focal length aerial telescopes). This optical design is now considered obsolete since with today's shorter focal length telescopes the eyepiece suffers from short eye relief, high image distortion, chromatic aberration, and a very narrow apparent field of view. Since these eyepieces are cheap to make they can often be found on inexpensive telescopes and microscopes.