Two thin prisms are combined to form an achromatic combination. For prism I, `A=4^(@),mu_(R)=1.35, mu_(Y)=1.40,mu_(v)=1.42` For prism II, `mu_(R)^(')=1.7, mu_(Y)^(')=1.8 and mu_(V)^(')=1.9` . Find the prism angle of prism II and the net mean deviation.