The domains structure of cobalt, gadolinium and terbium have been investigated; single crystals of gadolinium and terbium, were obtained in which the oxide content was considerably reduced by a solid state electrolysis technique. The Bitter technique has been used for most of the observations with a modified colloid, but a dry colloid technique has been used at temperatures below 240 K or higher than room temperature. Two different pieces of apparatus were designed for use at high or low temperatures. In the case of cobalt single crystals the results obtained at room, temperature are similar to those obtained previously. The pattern at 77 K was as expected from the anisotropy data. The lengths of daggers of reverse magnetization and the widths of their bases were found to be in direct proportion (for simple and complex daggers). The variation of the exchange constant A with temperature was also determined. It was not possible to observe a closure domain structure, however this did not rule out the possibility of a partial structure being present on the basal plane, though the variation of dam in width with temperature agrees best with that of Kittel model. The domain, structure of gadolinium was much easier to analyse than that of cobalt. At 274 K patterns on planes containing the c-axis shoved parallel l80 walls, with the development of partial closure structures at the surface near the basal plane and at the oxide inclusions. Basal plane patterns indicate that the l80 vails are not plane and give more detailed information about the mixed nature of the basal structure. The change in domain structure has been studied as the temperature is reduced to 77 K and under the influence of magnetic fields up to l400 Oe. Unexpected results were obtained when the normal field produced by a permanent magnet was applied to the basal plane. Such arrangements produced the well known honeycomb structure which was observed on other materials after an applied field of few K.Oe. was applied parallel or perpendicular to the c-axis. This was believed to be the result of a closure structure with magnetisation laid freely in basal plane. The non-uniformity of' the lines of force produced by such magnet will re-orient the direction of the magnetic moment within the closure region. The domain structure in a cube shaped terbium single crystal was observed at 210 K and different possibilities for the internal configuration were given. Agreement between experimental observation and the proposed model which consists of plate type domains magnetized along the b(_1) and b(_2) axis but not along the third axis b(_3)-axis, which is perpendicular to one surface. The equilibrium, width was measured and compared with the calculated one and they were in. good agreement. The structure at lower temperatures was studied. However as in gadolinium it is still not clear whether the pattern observed at low temperature represents a true domain structure
