What is "pole of a lens"

Pole \Pole\, n. [As. p[=a]l, L. palus, akin to pangere to make fast. Cf. Pale a stake, Pact.]

1. A long, slender piece of wood; a tall, slender piece of timber; the stem of a small tree whose branches have been removed; as, specifically:

   1. A carriage pole, a wooden bar extending from the front axle of a carriage between the wheel horses, by which the carriage is guided and held back.

   2. A flag pole, a pole on which a flag is supported.

   3. A Maypole. See Maypole.

   4. A barber's pole, a pole painted in stripes, used as a sign by barbers and hairdressers.

   5. A pole on which climbing beans, hops, or other vines, are trained.

2. A measuring stick; also, a measure of length equal to 5? yards, or a square measure equal to 30? square yards; a rod; a perch.
   --Bacon.

   Pole bean (Bot.), any kind of bean which is customarily trained on poles, as the scarlet runner or the Lima bean.

   Pole flounder (Zo["o]l.), a large deep-water flounder ( Glyptocephalus cynoglossus), native of the northern coasts of Europe and America, and much esteemed as a food fish; -- called also craig flounder, and pole fluke.

   Pole lathe, a simple form of lathe, or a substitute for a lathe, in which the work is turned by means of a cord passing around it, one end being fastened to the treadle, and the other to an elastic pole above.

   Pole mast (Naut.), a mast formed from a single piece or from a single tree.

   Pole of a lens (Opt.), the point where the principal axis meets the surface.

   Pole plate (Arch.), a horizontal timber resting on the tiebeams of a roof and receiving the ends of the rafters. It differs from the plate in not resting on the wall.