Dalek eyestalk
The eyepiece, also called the eyestick or similar names, was the part of the Dalek which enabled the Kaled mutant inside to see. Containing a wide-angle lens, the eyepiece could magnify images so that the Dalek could see across great distances and also track a person's footsteps. (DW: Destiny of the Daleks)
The eyepiece was the most vulnerable part of the Dalek. When it was taken out or covered, the Dalek no longer remained a threat. After the eyepiece is destroyed, its head would spin around uncontrollably and the Dalek's weapon would be fired in a blind panic, usually accompanied by the frantic screaming of "My vision is impaired— I cannot see!"
However, post-Time War Daleks overcame this exploitable weakness. For example, a Dalek was shot in the eyestalk by Wilf with a paintball gun. The paint was immediately melted off and the Dalek was heard to say "My vision is not impaired." (DW: The Stolen Earth) After the Daleks harnessed background radiation as a power source during the Last Great Time War, they developed a shield for their casings that would melt bullets, making the eyepiece more protected and harder to take out. (DW: The Parting of the Ways)
To escape from the Dalek base, the Doctor threw his hat onto a Dalek's eyestick and the Dalek began firing and destroying the command center, almost hitting Davros in the process until the Doctor destroyed it. (DW: Destiny of the Daleks)
To escape from Davros' base in Tranquil Repose, the Doctor took out a Dalek's eyepiece and Peri attached a grenade to its casing, destroying the Dalek. (DW: Revelation of the Daleks)
During the Shoreditch Incident, Ace fired an ATR (Anti-Tank Rocket) at an Imperial Dalek and was shocked at the damage. She responded by saying "I aimed for the eyepiece." While inside a Dalek shuttle craft, the Doctor placed his hat on a Dalek's eyepiece to confuse it. (DW: Remembrance of the Daleks)
The viewpoint from the eyepiece varied over time, from a simple telescope-like view (DW: The Dalek Invasion of Earth) to an infrared scanner (DW: Destiny of the Daleks) to a bright green (or white, in the case of Imperial Daleks) lens, with Kaled script readouts displayed in the top and bottom left corners. (DW: Remembrance of the Daleks) By the time of the Last Great Time War, the eyestalks all contained blue glowing lenses, with a targeting aid "on-screen" at all times. (DW: Dalek)
The Daleks produced by the Progenitor had radically redesigned eyepieces with black rings on the stick, fins on the cone, and a raised lens with a blue centre and orange rim. The view from within was much the same as from without, albeit with visible wires and a rectangular shape in the middle.
