The Nikon Illuminator

The illuminator for the Nikon SP is one of those strange gadgets that optical firms had great fun turning out in small quantities in the 1950s. One looks at it and says "How neat, or cool, or something …"
What does it do? It shines a small amount of light onto an SP's bright-frames window to allow the user to see the lens framelines even in low light levels.
The accessory consists of the illuminator and a small leather case which holds the device and up to two extra AA batteries. The illuminator consists of a black plastic housing that holds one AA battery. The plastic is similar to bakelite but softer. An accessory shoe sticks out the back, allowing the user to slide the device onto the SP's shoe from the front. A spring clip holds it in place. The shoe incorporates a pin connector to transfer the hot-shoe-sync from the camera to the top shoe on the illuminator. Below the housing is a swing switch and next to it a tiny port for a grain-of-wheat bulb. "Nippon Kogaku" appears in raised letters in front of the shoe.
In operation, you mount the illuminator and rotate the variable switch from the marked "off" position to the light level you are comfortable with.
The illuminator was inexpensive, but only a few SP owners seem to have ever bought one, and it lingered on Nikon price lists until the mid-60s.
The illuminator has two design problems. The end-cap for the battery compartment is a hard chrome that screws into the plastic with a side metal contact to get in the way of proper threading. As a result, cross-threading is an ever-present danger, and once the plastic is recut, the cap is almost impossible to get on correctly.
The other problem is that the rotating switch has no positive "off" click position. In practice, it is all too easy to hit the switch and leave the light on without realizing it. The inevitable result is a leaked battery and ruined compartment. Many illuminators suffered from the problem and got tossed. As a result, the illuminator today is a rare collectible.

The picture in the background of the illustration is the entrance of the Flicker Hole in the Guadalupe Mountains of New Mexico.