continuously supplying the polishing medium onto the sample surface the samples are polished by rotating brushes 33 that advantageously travel at the same speed as the conveyer belt fig4 shows a third polishing apparatus here the samples 40 can be held upside down on the bottom of vacuum suction holder 41 which is then lowered onto a rotating polishing pad or brush 42 wet with the polishing medium via tube 43 alternatively the sample can be placed on the bottom of the sample holder by mechanical means or by gluing fig5 shows an alternative polishing apparatus particularly useful for polishing non planar surfaces such as lenses the apparatus comprises a sample holder 50 such as a vacuum holder for holding a lens 51 a tube 52 for delivering the polishing medium and a polishing element 53 such as a rotating brush the invention can be more clearly understood by consideration of the following specific example a cvd diamond film deposited on a smooth si substrate was cut into 1 cm u00d7 1 cm squares the film had a surface roughness of u02dc 90 u00e5 it was polished by using a slurry mixture consisting of about 25 volume percent of mn powder average particle diameter less than 45 u03bcm and about 75 volume percent of 10 hydrochloric acid in distilled water a rotating brush u02dc 300 rpm was used to rub the polishing medium against the diamond.