Stroboscopic cupola was a historical

tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engin ...

cupola design consisting of armoured outer and inner

cylinders A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...

, both pierced by several regularly spaced vertical vision slits around the circumference. Rapidly rotating the external cylinder (in case of the

Char 2C The Char 2C, also known as the FCM 2C, was a French heavy tank, later also seen as a super-heavy tank. It was developed during World War I but not deployed until after the war. It was, in total volume or physical dimensions, the largest opera ...

tank design at 300 rpm with an electric motor) created the visual illusion of seeing through the cupola as if not there due to human

persistence of vision Persistence of vision traditionally refers to the optical illusion that occurs when visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been d ...

, similar to how a plank fence with alternating planks and holes fades from view when the observer moves alongside it at a speed. The short time both the inner and outer slits were perfectly aligned at any given moment still afforded protection of the inside space, as any projectile not capable of piercing the armour directly would need hit the spaced out slits and to come at an optimum angle and timing to go through both outer and inner slit, while it still allowed a better view of the surroundings than just a single vision slit or periscope with a very narrow field of view.

References

{{reflist Optical illusions