The ratio of the specific charge (e/m) of a proton to that of an $\alpha$  particle is

${\text{He}}^{+2}\to \frac{\text{e}}{\text{m}}=\frac{\text{Charge of prton}+\text{charge of neutron}}{\text{Mass}\text{of}\text{protons}+\text{mass}\text{of}\text{neutrons}}=\frac{2}{4}=\frac{1}{2}$      $\therefore 1:\frac{1}{2}$