  By using a method, previously established to calculate electromagnetic
fields, we compute the force of light upon a metallic particle. This procedure
is based on both Maxwell's Stress Tensor and the Couple Dipole Method. With
these tools, we study the force when the particle is over a flat dielectric
surface. The multiple interaction of light between the particle and the surface
is fully taken into account. The wave illuminating the particle is either
evanescent or propagating depending an whether or not total internal reflection
takes place. We analyze the behaviour of this force on either a small or a
large particle in terms of the wavelength. A remarkable result obtained for
evanescent field illumination, is that the force on a small silver particle can
be either attractive or repulsive depending on the wavelength. This behaviour
also varies as the particle becomes larger.
