It is generally believed that the structure of dust tails of comets can be described in terms of the mechanical theory. The basic parameters of this theory are the repulsive force and initial emission velocity of particles. However, the effect of initial velocity does not seem to have been considered seriously in the discussion of the overall structure of dust tails. Finson and Probstein (1968) have recently proposed an elaborated method to analyse the tail brightness profiles by introducing the distribution function of particles. The initial velocity effect was, however, taken into account only in an approximate way. They assumed that a group of particles are emitted isotropically with a single speed, and afterwards forms a spherical shell expanding uniformly in time. The isotropic emission is suggested by the fluid-dynamical considerations on the gas-dust interaction in the inner head region, but the subsequent spherical expansion implies merely a simplification for the calculation to obtain the line-of-sight integrals analytically. Due to this simplification, the validity of the Finson-Probstein method is seriously limited, particularly when a tail after perihelion passage is concerned.