AT the request of the Ministry of Munitions the Optical Society held an, exhibition of workshop methods of optical testing at King's College, Strand, on January n, in order that by the interchange of workshop methods of test, the production of optical instruments for naval and military use might be expedited. Amongst others, Messrs. Chance Bros, exhibited a method for the rapid approximate assessment of strain existing in glass. A plate of mica is cemented between glass plates, the mica being of such thickness as to give a phase difference in the two beams of one wave for sodium light. This plate therefore gives approximately the sensitive first order purple colour between crossed Nicols. According to the orientation of the specimen double refraction will be evident from the change of the purple colour to a tint of a lower or higher order. Each tint corresponds to a definite phase variation produced by the double refraction of the glass, and hence an estimation of the tints exhibited gives an estimation of the phase difference produced in a beam on passage through the glass. The colours given in conjunction with the wave plate are independent of the intensity of the light; thus greater uniformity in testing for bad annealing is obtained than by the use. of crossed Nicols alone, where the sensitiveness of the tests depends largely on the intensity of the source of light.