Accession No

6685

Brief Description

visibility meter, designed by Ernest Gold, by Casella, English, 1943

Origin

England; London; Fitzroy Square; Regent House

Maker

C. F. Casella and Company Ltd.

Class

meteorology

Earliest Date

1943

Latest Date

1943

Inscription Date

Material

rubber; metal; plastic

Dimensions

83mm (depth) x 77mm (height) x 265mm (width, retracted) [453mm wide with slide extended]

Special Collection

Michael J. Clark Bequest

Provenance

Inscription

HOLD IN LEFT HAND
CASELLA LONDON
MK II 1418/43

(inside slide near handle): 2035

Description Notes

visibility meter, designed by Ernest Gold, by Casella, English, 1943

An instrument used to measure visibility at night. The observer would require a series of fixed lights of known power and distance as a reference; he would then look through the rubber accordion eyepiece and adjust the slide until the lights became obscured. By reading the metre he could ascertain visibility over various distances, measured in "nebules," a visibility unit devised specifically for this instrument (now obsolete). The slide contains two "wedges": transparent, gradually tinted panes which, according to the 1948 Casella catalogue (in OHF), are of "neutral density"; along with a small compensating wedge just behind the eyepiece, they "produce a field of view of uniform density at any position of the wedge."

The rubber accordion eyepiece can be adjusted to view through an upper and lower section of the wedge, providing two ranges: the lower from 10-120 nebules, the upper from 110-220 nebules. Graduated white plastic scales run alongside both wedge sections, allowing the user to read the instrument. Ultimately, the reading would be read against an index, perhaps to indicate safe visibilities for landing and taking off at an airbase. Catalogue notes that the instrument was designed by Ernest Gold, Esp., F.R.S., and is to the specification of the British Meteorological Office.

Complete.

The meter is in a hinged wooden box with two clasps and felt-lined racks to keep the instrument in place. Also in the box is an oblong cylindrical piece of metal used to shield the other end of the eyepiece (?), as well as a small pane of plastic.

References

https://www.analogweather.com/uploads/7/7/7/5/77750690/casella_-_meteorological_and_scientific_instrument_catalog_684_-_1948.pdf

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