origanum-alcohol (1 part absolute alcohol to 3 parts origanum oil) and placed on a slide in a layer of origanum oil, blotted and mounted in balsam. Should sections become milky in the oil, renew the latter until they are cleared. 4. =Sectioning of Paraffin Blocks.= The paraffin block trimmed to the proper proportions, leaving a rim of paraffin about 2-3 mm. wide all about the tissue, is fastened to the wooden block by melting the under side of the paraffin by drawing a hot knife across it, and then immediately pressing the block upon the wood. The wooden block is then clamped into the object-holder. If desired the paraffin block can be fixed directly to the metal plate of the object-holder in the same manner. The block is raised until the level of the edge of the knife is reached. The knife is placed transversely or at a slight angle, and the cutting done with a relatively small portion of the edge. The paraffin block may be breathed upon to warm slightly the upper layer when a hard paraffin is used, and the knife is then drawn carefully through the block, guiding the section onto the knife-blade by means of a fine-pointed camel’s-hair brush held in the left hand. It is preferable, I think, to have a wider rim of paraffin at one corner of the block and to place the block with that corner at a slight angle to the knife, so that the edge of the blade will first engage the block at that point. The tip of the camel’s-hair brush moistened in water catches the corner of the section as the knife begins to cut, and pushes it over onto the blade, thus holding the section flat and preventing curling. When entirely cut through, the section is removed from the knife by the brush, still holding it at the corner first touched, or if necessary it is again moistened and applied to the upper side of the section, lifting the latter off the knife. The block is trimmed down to the proper level by cutting thick sections first, then sections of the desired thickness when the level of the entire block is reached. The sections are then transferred, with their shiny side down, just as they come off the knife, to a slide, sheet of paper, warm water, warm molasses or sugar-dextrin solution, or 70 per cent alcohol, and treated further according to the directions given in the next chapter. The presence of ridges on the cut surface of the block is an indication that the knife needs stropping. The knife must cut the paraffin, not scrape it. Crumbling of the paraffin is the result of imperfect infiltration during the imbedding process; water, alcohol, aniline oil or xylol may be left in the tissue, the paraffin may contain oil, or the cooling process may have been delayed. Curling of the sections can be prevented by using a sharp knife and slightly warming the surface of the block by breathing upon the block, use of a warm knife or spatula, heat-focus, etc. For ordinary work paraffin sections 5-7µ thick are easily obtained; for especial work 1-2µ sections may be