Polarised light micrograph of a feather from an unidentified bird. A feather consists of a central axis or rachis, continuous with the hollow quill attached to the body. The rachis bears the flat vane of the feather which is made up of many slender barbs bearing barbules along each side. The barbules of adjacent barbs interlock to form a continuous surface. Magnification: X18 (at 35mm size).