We report here the spatial distortion of a dye laser beam on propagati
on through a column of copper vapour. With the dye laser frequency tun
ed on the red side of the copper resonance line (578.2 nm), we have ob
served self-defocusing of the beam. This was caused by the resonant no
n-linearity associated with the electronic polarizability of the atomi
c vapour. From our data on self-defocusing we have measured the nonlin
ear refractive index n(2) for radiation red detuned with respect to th
e copper atomic transition and found it to be in good agreement with o
ur calculated value of n(2).