A chromatic method of making optical settings

The paper describes a visual modification of a previously described photo-electric method of measuring small angles. Great simplification is introduced by the substitution of visual for photo-electric equipment at the sacrifice of sensitivity. Angular displacements are measured by means of a hollow prism previously described. The settings are made by causing an image of a fine slit to fall on the sharp edge of a steel prism. The two components of the divided beam, after being coloured respectively red and green, are recombined, and a visual method is employed to observe the resulting colour. The relative position of the prism edge and image determines the proportion of red and green and is very sharply defined by the position of best resultant yellow. The settings have average errors of 0.2 second and are less fatiguing than cross-wire settings.