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 δ1 and angular dispersion Δθ, both
δ1 = A1(µ1–1) ...(i)
Δθ1 = A1(µV1–µR1) ...(ii)
Now the light beam again suffer deviation and dispersion by flint glass prism
δ2 = A2(µ2–1) ...(iii)
Δθ2 = A2(µV2–µR2) ...(iv)
Condition for no dispersion
Δθ1+Δθ2= 0
A1(µV1–µR1) + A2(µV2–µR2) = 0
A2 = - A1

(µV1−µR1)

(µV2−µR2)

Negative sign shows that two prisms must be placed with their base on opposite sides.
Net deviation in this condition

δnet = δ1+δ2
δnet = A1(µ1–1)+Α2(µ2–1)
(b) Dispersion without deviation
Net deviation should be zero
δnet = δ1+δ2
0 = A1[µ1–1]+Α2[µ2–1]
Thus, A2 = - A1

[µ1−1]

[µ2−1]

Here again negative sign shows that two prisms must be placed with their base on opposite sides.
Net angular dispersion
Δθ = Δθ1+Δθ2 ⇒ Δθ = A1 (μV1–μR1)+A2(μV2–μR2)