For example, the marginal flanges 614 and 616 of the support shown in FIG. 17 could be tinted in a color such as blue, with the support being formed of a transparent material such as polyvinyl chloride or polysulfone, and with the ridge of the plate being complementarily tinted with a yellow color, so that proper superpositioning of the support on the plate with the marginal flange overlying the r