On the dispersion of artificial double refraction

1. It is well known that glass compressed unequally in different directions behaves like a crystal whose optic axis is along the line of stress. If T1 , T2 are the principal stresses in the wave front,μ 1 ,μ 2 the refractive indices of the two rays for which the directions of vibration are along T1 , T2 respectively, then the relative retardation of the two oppositely polarised rays is R = (μ 1 —μ 2 )τ = C (T1 — T2 )τ , whereτ is the thickness of glass traversed. C may be called the “stress-optical coefficient ” of the glass. It differs for different glasses and in the same glass for different colours, but it is usually assumed independent of the value of the stress.