E.R. Watts & Son, London, Cotton Type Rangefinder

The E.R. Watts & Son, London, Cotton Type Rangefinder.
Figure 1: The E.R. Watts & Son, London, Cotton Type Rangefinder.
Like the Schick Incorporated stadimeter this instrument was used to measure the distance between an object and the observer.

A 4" long linear concave lens (in cross section front and rear of the glass are parallel) is fixed to the base. A prism is mounted on a carriage with an index mark that slides back and forth along the axis of the instrument. When at infinity the slanted side of the prism is parallel to the corresponding part of the concave lens and cancels out any light breaking.

The distance to an object can be measured when the height (or width) of it is known. The carriage is slid until both ends of the object coincide, similar to the use of the 1943 U.S. Navy stadimeter or sextant.

The Cotton Type Rangefinder from the other side.
Figure 2: The Cotton Type Rangefinder from the other side.
Three reversible brass inserts calibrated on both sides in yards, correspond to various base heights, (20 and 25 feet, 30 and 35 feet and 40 and 50 feet). They are fitted along the axis of the instrument. When the scale for the appropriate base height is selected and inserted, the carriage can be slid for coincidence and the distance read at the index mark on the sliding carriage.

Notes

For more information see:
- Mariner's Museum has a slightly different and newer version of the instrument in their collection.
- Peter Ifland, Taking the Stars, Newport News, 1998, p.150.


If you have any questions and/or remarks please let me know.

The box of the Cotton Type Range Finder.
Figure 3: The box of the Cotton Type Range Finder.
 
The box of the Cotton Type Range Finder opened, revealing its contents.
Figure 4: The box of the Cotton Type Range Finder opened, revealing its contents.

The lens and prism of the Cotton Type Range Finder.
Figure 5: The lens and prism of the Cotton Type Range Finder.
 
The prism can be swung away for cleaning.
Figure 6: The prism can be swung away for cleaning.

The label in the box explaining how to change the scales.
Figure 7: The label in the box explaining how to change the scales.
 
The three labels (top) have scales on both sides (bottom).
Figure 8: The three labels (top) have scales on both sides (bottom).

The three scales are inserted along the top end of the instrument.
Figure 9: The three scales are inserted along the top end of the instrument.
 
Each scale tells what is on the reverse.
Figure 10: Each scale tells what is on the reverse.

The indication "Range Finder Cotton Type Mk II".
Figure 11: The indication "Range Finder Cotton Type Mk II".
 
The manufacturer's name "E.R. Watts & Son, London".
Figure 12: The manufacturer's name "E.R. Watts & Son, London".

Reading the scale is done at the centre of the prism carriage (here at 980 yards).
Figure 13: Reading the scale is done at the centre of the prism carriage (here at 980 yards).
 
The E.R. Watt & Son, London, Range Finer Cotton Type Mk II on its box.
Figure 14: The E.R. Watt & Son, London, Range Finer Cotton Type Mk II on its box.

Celestial Navigation Coastal Navigation Distance measurement
1943 U.S. Navy stadimeter Cotton Type Range Finder