At night there is insufficient light to see the reticule. So they had to be illuminated by a small lamp. This could be clicked on. On the connector is a small knob which moves a v-shaped slit in order to control the amount of light. The plug at the other end of the cable had different pins. Normaly the lamps run on flat 4,5 V batteries, which were placed with three in a small steel or leather box. Almost all optics with cross-hairs have a light connector. Rangefinders, tripods and MG mountings all have a clamp or straps for mounting the batterie box. More complex optics such as rangefinders, had more than one cable. For full control there was a junctionbox for three cables with a voltage regulator. These small lamps were also used for the illumination of dials and numbered scales on many type of weapons.
An illumination cable made by Busch. In the detail the little knob and the v-slit to temper the light. Sometimes we find a small red filter inside.
In leather and in steel. Two copper plates inside make contact with the springs of the battery. The internal plate is made of bakelite.
The control box for connecting three lamps. One was controlled with the knob.
During sunshine the incoming light had to be tempered. Extra sunglasses could be placed on the oculairs. Other optics had built in filters which were controlled with a knob (Farbgläser).
External filters come in two grades of darkness. Spare bulbs and sunfilters in a neat box.
The internal filters of a 12 x 60.
In cupolas only low voltage was used. Here a connector for the illumination cable of an optical device.