Chapter 27 - Optical Instruments - Problems and Conceptual Exercises - Page 974: 106

$M=1+\frac{N}{f}$

We know that $\frac{1}{d_{\circ}}=\frac{1}{f}-\frac{1}{d_i}$ The viewer's near point is $=N$ Thus, multiplying both sides by $N$, the above equation becomes $\frac{N}{d_{\circ}}=\frac{N}{f}-\frac{N}{d_i}$ $\implies M=\frac{N}{f}-\frac{N}{d_i}$ $\implies M=\frac{N}{f}-\frac{N}{-N}$ because $d_i=-N$ $\implies M=\frac{N}{f}+1$ or $M=1+\frac{N}{f}$