b
$a=$ Main Scale division
$b=$ vernier scale division
$L . C .=\left(\frac{ b - a }{ b }\right) M$
$=\frac{50-49}{50} M$ $M=$ Main scale reading
$L C =\frac{ M }{50}$ $M =7.05-7.00$
$L C \cdot=\frac{0.05}{50} \quad M =0.05$
L.C. $=0.001$
Reading $=7+0.001 \times 23=7.023 cm$