It is well that the optician should know how to test optical glass for its various faults, although the optical glass made in this country is so good that it is not necessary in the ordinary course of manufacture to test it in its raw state. If the glass-maker is concerned to determine the origin of dead metal or "stones", the nature of these can be determined by X-ray diffraction. Veins are threads or streaks of glass within the mass having a refractive index so different from the bulk of the glass as to be distinguishable to the eye. The degree of double refraction varies with the strain from point to point of the glass. Glass which has not been sufficiently well-annealed after the chilling which takes place in moulding shows double refraction, but the effect can be quite marked on the strain viewer without in itself causing optical parts made from the glass to be defective.