Answer
$0.325mm$
Work Step by Step
The distance between the objective lens and the object is
$d_{\circ}=(\frac{1}{f_{objective}}-\frac{1}{d_i})^{-1}$
We plug in the known values to obtain:
$d_{\circ}=(\frac{1}{0.0324}-\frac{1}{15.38})^{-1}$
$d_{\circ}=0.325mm$
