The two element Huygens
eyepiece was invented by Christiaan Huygens
in the 17th century. This optical design is now considered obsolete. Their main use in optics is as an example of the simplest possible compound lens design.
Despite being deprecated, these eyepieces are inexpensive to make and so are often sold with the cheapest telescopes and microscopes. Huygens eyepieces suffer from short eye relief, high image distortion (especially on short focus telescopes), chromatic aberration and have very narrow apparent field of view.
Essentially their only good use is for projection of a solar image onto a screen. Because Huygens eyepieces do not contain cement to hold the lens elements, they are less likely to be damaged by the intense, concentrated light of sun. Lens cement can overheat and either dissolve or burn.
Huygens eyepieces consist of two plano-convex lenses
with the plane sides towards the eye separated by an air gap. The lenses are called the eye lens and the field lens. It is usually designed for zero transverse chromatic aberration. The focal plane is located between the two lenses. If the lenses are made of glass of the same refractive index, to be used with a relaxed eye and a telescope with an infinitely distant objective then the separation is given by:
where fA and fB are the focal lengths of the component lenses.