  An experiment is proposed to excite the 'forbidden' 1s2s 3S1 - 1s2s 1S0
magnetic dipole (M1) transition at 1557 nm in a collimated and slow atomic beam
of metastable helium atoms. It is demonstrated that an excitation rate of 5000
/s can be realised with the beam of a 2W narrowband telecom fiber laser
intersecting the atomic beam perpendicularly. A Doppler-limited sub-MHz
spectroscopic linewidth is anticipated. Doppler-free excitation of 2% of
trapped and cooled atoms may be realised in a one-dimensional optical lattice
geometry, using the 2W laser both for trapping and spectroscopy. The very small
(8 Hz) natural linewidth of this transition presents an opportunity for
accurate tests of atomic structure calculations of the helium atom. A
measurement of the 3He - 4He isotope shift allows for accurate determination of
the difference in nuclear charge radius of both isotopes.
