Weston Leicameter Exposure Meter Model 650: 1938, USA manufacture.
The Leica was a landmark development in camera design. The first precision stills camera to use 35mm cine film, it had a cloth focal plane roller-blind shutter. Generally available from 1932, it became an immediate worldwide success. The construction of the shutter lead to a range of shutter speed settings that did not follow the conventional progression common to leaf shutters and was, in parts, non-linear.
Weston made this version of the model 650 to support the new camera. Instead of Candles / Square Foot or relative brightness, the meter reads shutter speeds directly, for a reference point of f6.3 and 23 degrees Scheiner. As far as I know, the Leicameter was the only Weston to use the Scheiner ratings, then common in Germany. A model 650 Leicameter with Weston Speeds was also made in fewer quantities.
The film speed is set on the dial, which is then rotated to align the pointer to the shutter speed indicated on the meter. other combinations of shutter speed and aperture can then be read off.
A wide range of shutter speeds are marked on the dial. Early Leicas had a top shutter speed of 1/500th sec. 1/1000th is marked on the meter. Standard and model II Leicas did not have slow shutter speeds below 1/20th sec. But by 1938 the Model III Leica with speeds down to 1 second was in widespread use (for those who could afford it).
Optimistically, the shutter speed dial shows time exposure speeds down to 64 seconds. From about 4 seconds down, these speeds would be wildly inaccurate as films exhibited considerable apparent reductions in sensitivity as the seconds of exposure clocked up. This well-recognised condition is known as Reciprocity Failure. The experienced photographer would have needed to increase his exposures significantly in this time range to obtain a useable negative.
As well as the 650 Leicameter marked up for degrees Scheiner, Weston also made a smaller number of a variant based on the more familiar Weston film speed ratings. Visually this variant differs from the Scheiner model in that the emulsion speed window on the dial reads 'Emulsion Speed' in place of 'Degrees Scheiner'. On the meter face the film speed reference reads 'Weston'.