Baden-Powell's unilens

The unilens monocular is a simple refracting telescope for field use, designed by Robert Baden-Powell.

Consisting of only one lens, it is the simplest of all telescopes, and while occupying very little space can still magnify a distant image up to about four times.

[1][2] The unilens monocular is simply a 2.5 inch (6.4 cm) diameter convex lens with a 6 foot (1.8 m) focal length, equivalent to +0.55 dioptres.

Prolonged use is furthermore tiring to the eye and thus a unilens is mainly suited for occasional or intermittent viewing of distant objects.

Original instructions advise the user to sit with the arm holding the device resting on the knee and to move the lens forward and back until the desired object is in focus.