In building a complete system for the Nikon F camera, Nikon provided a way to illuminate the Meter Needle
of Photomic meters when the camera was used in darkness.
The light source lays on top of the finder and is secured to it by screwing its retaining ring to the circular
eyepiece. Early Photomic meter/prisms had square eyepieces that did not have a way to attach accessories such as
diopter-correcting eyepieces or the DL-1 Illuminator.
The DL-1 has a larger "on" button on the top and a smaller "off" button on the side.
The DL-1 uses the same type of battery that the Photomic
meters use, either the original 1.35 volt Mercury battery or the 1.5 volt alkaline or sliver oxide batteries that
replaced the Mercury type when its sale in the U.S. was stopped due to environmental concerns.