  The work to be described falls into three parts:
  (1) The computer-controlled CW spectrometer was designed to supplement the
Manchester pulsed microwave spectrometer in situations where rapid nuclear
relaxation makes spin-echo spectroscopy difficult. Its operating range is 4-8
GHz. Resonator designs and modulation strategies will be discussed in the light
of practical experience.
  (2) Both CW and pulsed NMR have been used to study the field dependence of
the hyperfine splittings of $^{165}$Ho in HoF$_3$ and, as a dilute substituent,
in YF$_3$. The low site symmetry results in a singlet crystal-field ground
state for the Ho$^{3+}$ ion, giving Van Vleck paramagnetism and enhanced
nuclear magnetism at low temperatures. The measurements were made at
temperatures in the range 1.5 to 4.2 K and in fields of up to 8 T. This work
has revealed, for the first time, distinct spectra from the two subtly
inequivalent rare-earth sites in the orthorhombic unit cell. Because of the
non-colinear spin structure of HoF$_3$, the NMR and magnetometry measurements
give independent and complimentary information about the ionic moments.
  (3) The measured hyperfine splittings have been interpreted in terms of a
15-parameter crystal-field Hamiltonian appropriate to the $C_{1h}$ site
symmetry. This work has entailed a substantial effort to clarify the notational
confusion that exists in the literature. A computer program has been developed
to automate conversion between notational conventions prior to diagonalization
of the 136-dimensional electronic-nuclear Hamiltonian comprising the Zeeman,
crystal-field and hyperfine interactions.
  [abridged]
