0 a device, usually held away from a camera, that automatically makes a flash when a camera is taking a picture --
An external electronic flashgun was also available.
About a year later, a threaded hole was added at the right-hand front above the flashbulb synchronising contacts for securing the flashgun to the camera (version 4).
Observers were required to verbally report how many disks had been presented, both at 10 s and at 60 s after the flashgun exposure.
The 100/104 designation refers to the type of flash: models ending in 0 had a built-in flashgun, while those ending in 4 used flashcubes.
Using a flashgun to illuminate a line of white disks, they were able to generate intense afterimages in dark-adapted observers.