[![enter image description here][1]][1]


$\color{red}{\delta^{*}}$=minimum deviation angle.

$\color{red}{\mathrm{i}1^{*}}$=incident angle for minimum deviation. 

$\color{blue}{\mathrm{i}1^{\boldsymbol{+}}}$=incident angle $\mathrm{i}1^{*}$ plus a variation  $\:\theta$. 

$\color{green}{\mathrm{i}1^{\boldsymbol{-}}}$=incident angle $\mathrm{i}1^{*}$ minus a variation  $\:\theta$.

$\color{blue}{\delta^{\boldsymbol{+}}}$= deviation if incident angle equals $\mathrm{i}1^{\boldsymbol{+}}$.

$\color{green}{\delta^{\boldsymbol{-}}}$= deviation if incident angle equals $\mathrm{i}1^{\boldsymbol{-}}$.

Result : $\delta^{\boldsymbol{+}}\ne\delta^{\boldsymbol{-}}$.


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The drawing is sketched with the following data and calculations:
\begin{align}
\mathrm A & = \text{prism angle}= 60^{\rm o} 
\tag{01}\\
n_{1} & = \text{refraction index of surroundings}= 1.00
\tag{02}\\
n_{2} & = \text{refraction index of prism}= 1.50
\tag{03}\\
n & = \text{relative refraction index}=1/1.50=0.6667
\tag{04}\\
\mathrm i1^{\boldsymbol{*}} & = \text{incident angle of minimum deviation}= \arcsin\left(n\sin\tfrac{\mathrm A}{2}\right)=48.59^{\rm o} 
\tag{05}\\
& \text{minimum deviation path (red) }  : 48.59^{\rm o} \Longrightarrow 30^{\rm o} \Longrightarrow 30^{\rm o}  \Longrightarrow  48.59^{\rm o}
\tag{06}\\
\delta^{*} & = \text{minimum deviation angle}=2\cdot\mathrm i1^{\boldsymbol{*}}-\mathrm A=37.18^{\rm o} 
\tag{07}\\
\theta & = \text{variation angle}=20^{\rm o}  
\tag{08}\\
\mathrm{i}1^{\boldsymbol{+}} & = \mathrm i1^{\boldsymbol{*}}+\theta=68.59^{\rm o} 
\tag{09}\\
 \mathrm{i}1^{\boldsymbol{+}} & \text{ deviation path (blue) }  : 68.59^{\rm o} \Longrightarrow 38.36^{\rm o} \Longrightarrow 21.64^{\rm o}  \Longrightarrow  33.58^{\rm o}
\tag{10}\\
\delta^{\boldsymbol{+}} & = \text{deviation angle if incident angle equals }\mathrm{i}1^{\boldsymbol{+}} =42.17^{\rm o} 
\tag{11}\\
\mathrm{i}1^{\boldsymbol{-}} & = \mathrm i1^{\boldsymbol{*}}-\theta=28.59^{\rm o} 
\tag{12}\\
 \mathrm{i}1^{\boldsymbol{-}} & \text{ deviation path (green) }  : 28.59^{\rm o} \Longrightarrow 18.60^{\rm o} \Longrightarrow 41.40^{\rm o}  \Longrightarrow  82.70^{\rm o}
\tag{13}\\
\delta^{\boldsymbol{-}} & = \text{deviation angle if incident angle equals }\mathrm{i}1^{\boldsymbol{-}} =51.29^{\rm o} 
\tag{14}
\end{align}


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[![enter image description here][2]][2]
[![enter image description here][3]][3]
[![enter image description here][4]][4]


  [1]: https://i.sstatic.net/blKdi.png
  [2]: https://i.sstatic.net/TBF1Y.png
  [3]: https://i.sstatic.net/pMiUN.png
  [4]: https://i.sstatic.net/gTj2m.png