Here we find that a fully immersed low refractive index SiO2 microsphere (or a microcylinder, a yeast cell) can clearly distinguish a sample with sub-diffraction features in dark-field illumination mode. The resolvable area of the sample by microsphere-assisted microscopy (MAM) is composed of two regions. One region locates below the microsphere, and a virtual image of this part of the sample is formed by the microsphere first and then the virtual image is received by the microscope. The other region is around the edge of the microsphere, and this part of the sample is directly imaged by the microscope. The simulated region of the enhanced electric field on the sample surface formed by the microsphere is consistent with the resolvable region in the experiment. Our studies show that the enhanced electric field on the sample surface generated by the fully immersed microsphere plays an important role in dark-field MAM imaging, and this finding will have a positive effect on exploring novel mechanisms in resolution improvement of MAM.