$\color{red}{\delta^{*}}$=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 : $\delta^{\boldsymbol{+}}\ne\delta^{\boldsymbol{-}}$.