Experiments have been performed to investigate the useful thickness limits for electron transmission under routine operating conditions for silicon and stainless steel. The criterion adopted for this limit was that thickness at which fringe contrast at faults was destroyed by absorption. The results indicate a roughly linear behaviour of this limit at intermediate voltages, then falling off, but less rapidly than a (v/c)2law. At 1 Mev foils of silicon ∼9 μ and stainless steel ∼2 μ thick are adequately transparent for observing defects. The ultimate thickness limit is determined by contrast and chromatic aberration.