They offer three lens colors with their frames--a luminator, a neutralizer and a deflector.
The deflector is an instrument designed to enable an observer to reduce the deviations of the compass to an amount not exceeding 2 0 during fogs, or at any time when bearings of distant objects are not available.
With the deflector any inequality in the directive force can be detected, and hence the power of equalizing the forces by the usual soft iron and magnet correctors.
Several kinds of deflector have been invented, that of Lord Kelvin (Sir William Thomson) being the simplest, but Dr Waghorn's is also very effective.
The use of the deflector is generally confined to experts.