The first part of this thesis presents a review of colour-magnitude photometry of Galactic globular clusters. A survey has been made of recently-published colourmagnitude\ud photometry of globular clusters, and a self-consistent distance scale established for these data, using the method of subdwarf matching to the observed main sequences. The distances thus derived have been combined with other published colour-magnitude data to infer cluster ages and horizontal branch magnitudes. The relationship between cluster horizontal branch magnitude and metallicity is dicussed.\ud \ud In Chapter 2, a review is made of the photometric applications of charge coupled devices, giving particular attention to the aquisition and analysis of crowded field CCD images within globular clusters. Digital photometric techniques are discussed, including analytic profile fitting to stellar images.\ud \ud The major experimental part of this thesis concerns observations of the globular cluster w Centauri. These observations, their subsequent reduction, and photometric analysis, are discussed in Chapter 3. This chapter also contains a discussion of previous work on this cluster, and highlights its extremely unusual nature. The significant discovery of geometrically varying stellar profiles in the\ud programme CCD images, and the techniques for their analysis, are discussed.\ud \ud The colour-magnitude data from these observations are discussed in Chapter 4. The distance to the cluster is determined, and it is shown that there exists an intrinsic spread in colour upon the cluster main sequence. Theoretical isochrones are employed to show that this spread in colour is consistent with a main sequence\ud metallicity distribution similar to that seen in more highly-evolved stars within the cluster. The age of the cluster is determined as 16 +3 Gyr, by comparison with\ud theoretical isochrones. This result is consistent with recent determinations for other clusters, and supports the theory that the Galactic globular cluster system is essentially coeval. The cluster main sequence luminosity function has been constructed, and is compared with recent observations and theory.\ud \ud In Chapter 5, a preliminary colour-magnitude study of the SMC cluster Lindsay 11 is presented.\ud \u
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