FIG. 1: Geometry of a NSOM fiber tip. It is characterized by i ) the aperture radius a in the metallic coating and ii ) the tip angle β . The quasi-multipole model is valid only in the far-field zone where a/ (sin β ) ≤ r . The boundary conditions cannot be applied in the near-field (or aperture) zone because it contains no metal. contains, as a first approximation, the two-dipole model anticipated earlier[6]. It justifies the generality of the Bethe’s result for the case of a conical screen. Our model is in good agree- ment with the experimental data and explains in particular the backward emission found for the P-polarization. The angular transmission profile does not depend on the aperture radius for small holes, and is insensitive to the tip angle as long as it is smaller than 20 $^{◦}$. [1] Rayleigh L., Philos. Mag., XLIV (1897) 28. [2] Bethe H. A., Phys. Rev., 66 (1944) 163. [3] Bouwkamp C., Philips Res. Rep., 5 (1950) 321. [4] Grober R. D., Rutherford T. and Harris T. D., Appl. Opt., 35 (1996) 3488. [5] Betzig E., Trautman J., Harris T., Weiner J. and Kostelak R., Science, 251 (1991) 1468. [6] Oberm¨ uller C. and Karrai K., Appl. Phys. Lett., 67 (1995) 3408. [7] Oberm¨ uller C., Karrai K., Kolb G. and Abstreiter G., Ultramicroscopy, 61 (1995) 171. [8] Jackson J. D., Classical Electrodynamics (J. Wileyand Sons, New York) 1975. [9] Bouwkamp C. J. and Casimir H. B. G., Physica, 20 (1954) 539. 7