What is "cobweb micrometer"

Cobweb \Cob"web`\, n. [Cob a spider + web.]

1. The network spread by a spider to catch its prey.

2. A snare of insidious meshes designed to catch the ignorant and unwary.

   I can not but lament thy splendid wit Entangled in the cobwebs of the schools.
   --Cowper.

3. That which is thin and unsubstantial, or flimsy and worthless; rubbish.

   The dust and cobwebs of that uncivil age.
   --Sir P. Sidney.

4. (Zo["o]l.) The European spotted flycatcher.

   Cobweb lawn, a fine linen, mentioned in 1640 as being in pieces of fifteen yards.
   --Beck. Draper's Dict.

   Such a proud piece of cobweb lawn.
   --Beau. & Fl.

   Cobweb micrometer, a micrometer in which threads of cobweb are substituted for wires.