By using two identical shape prism of crown glass and flint glass kept with their base on opposite sides, we can observe deviation without dispersion or dispersion without deviation. (a) Deviation without dispersion A white beam incident on crown glass will suffer deviation $\delta_1$ and angular dispersion Δθ, both $\delta_1$ = $A_1 (\mu_1 - 1)$ ...(i) $\Delta\theta_1$ = $A_1 (\mu_{V1} - \mu_{R1})$ ...(ii) Now the light beam again suffer deviation and dispersion by flint glass prism $\delta_2$ = $A_2 (\mu_2 - 1)$ ...(iii) $\Delta\theta_2$ = $A_2 (\mu_{V2} - \mu_{R2})$ ...(iv) Condition for no dispersion $\Delta\theta_1 + \Delta\theta_2$= 0 $A_1 (\mu_{V1} - \mu_{R1})$ + $A_2 (\mu_{V2} - \mu_{R2})$ = 0 $A_2$ = - $\frac{A_1 (\mu_{V1} - \mu_{R1})}{\mu_{V2} - \mu_{R2}}$ Negative sign shows that two prisms must be placed with their base on opposite sides. Net deviation in this condition $\delta_{\text{net}}$ = $\delta_1 + \delta_2$ $\delta_{\text{net}}$ = $A_1 (\mu_1 - 1) + A_2 (\mu_2 - 1)$ (b) Dispersion without deviation Net deviation should be zero $\delta_{\text{net}}$ = $\delta_1 + \delta_2$ 0 = $A_1 [\mu_1 - 1] + A_2 [\mu_2 - 1]$ Thus, $A_2$ = - $\frac{A_1 [\mu_1 - 1]}{[\mu_2 - 1]}$ Here again negative sign shows that two prisms must be placed with their base on opposite sides. Net angular dispersion Δθ = $\Delta\theta_1 + \Delta\theta_2$ ⇒ Δθ = $A_1$ $(\mu_{V1} - \mu_{R1}) + A_2 (\mu_{V2} - \mu_{R2})$