What is "spot lens"

Spot \Spot\ (sp[o^]t), n. [Cf. Scot. & D. spat, Dan. spette, Sw. spott spittle, slaver; from the root of E. spit. See Spit to eject from the mouth, and cf. Spatter.]

1. A mark on a substance or body made by foreign matter; a blot; a place discolored.

   Out, damned spot! Out, I say!
   --Shak.

2. A stain on character or reputation; something that soils purity; disgrace; reproach; fault; blemish.

   Yet Chloe, sure, was formed without a spot.
   --Pope.

3. A small part of a different color from the main part, or from the ground upon which it is; as, the spots of a leopard; the spots on a playing card.

4. A small extent of space; a place; any particular place. ``Fixed to one spot.''
   --Otway.

   That spot to which I point is Paradise.
   --Milton.

   ``A jolly place,'' said he, ``in times of old! But something ails it now: the spot is cursed.''
   --Wordsworth.

5. (Zo["o]l.) A variety of the common domestic pigeon, so called from a spot on its head just above its beak.

6. (Zo["o]l.)

   1. A sci[ae]noid food fish ( Liostomus xanthurus) of the Atlantic coast of the United States. It has a black spot behind the shoulders and fifteen oblique dark bars on the sides. Called also goody, Lafayette, masooka, and old wife.

   2. The southern redfish, or red horse, which has a spot on each side at the base of the tail. See Redfish.

7. pl. Commodities, as merchandise and cotton, sold for immediate delivery. [Broker's Cant]

   Crescent spot (Zo["o]l.), any butterfly of the family Melit[ae]id[ae] having crescent-shaped white spots along the margins of the red or brown wings.

   Spot lens (Microscopy), a condensing lens in which the light is confined to an annular pencil by means of a small, round diaphragm (the spot), and used in dark-field illumination; -- called also spotted lens.

   Spot rump (Zo["o]l.), the Hudsonian godwit ( Limosa h[ae]mastica).

   Spots on the sun. (Astron.) See Sun spot, ander Sun.

   On the spot, or Upon the spot, immediately; before moving; without changing place; as, he made his decision on the spot.

   It was determined upon the spot.
   --Swift.

   Syn: Stain; flaw; speck; blot; disgrace; reproach; fault; blemish; place; site; locality.

n. (cx microscopy English) A condensing lens in which the light is confined to an annular pencil by means of a small, round diaphragm (the "spot"), and used in darkfield illumination.