A thin prism P₁ with angle 4^o and made from glass of refractive index 1.54 is combined with another thin prism P₂ made from glass of refractive index 1.72 to produce dispersion without deviation. The angle of the prism P₂ is :

 3^o  

$ \delta = \delta_1 + \delta_2 = (\mu_1 - 1)A_1 + (\mu_2 -1 ) A_2 = 0.54\times 4^o + 0.72\times A = 0$

$ A = \frac{-2.16}{0.72} = -3^o$