Two thin prisms are combined to form an achromatic combination for 1^st prism $\mathrm{A}=4^0,{\mu }_R=1.35,{\mu }_v=1.42\text{ for }{2}^{\text{nd }}$ prism ${\mu }_R=1.7,{\mu }_v=1.9$ Find angle of 2^nd prism