The section should be examined frequently, and if it is noticed that one part is thicker than another, place the section on the lap so that the thicker part is towards the periphery, and put a little more pressure upon this part. Gutting or grooving the copper laps is prevented by moving the section from center to periphery and back, at the same time giving to it a slight rotary motion. It is best to have the lap revolve from right to left (in the opposite way from the hands of a watch), as, holding the section on the right side of the lap, it is easier to keep it in place by a pulling motion rather than by pushing it against the motion of the lap. In the final stages of grinding and polishing great precaution must be exercised, as one or two turns too many of the lap will often tear away a large part of the very thin section. The section should be frequently examined, by transmitted light, with a small, low power microscope, a drop of water or oil being put on the section to render it more transparent. If the section is thick and fastened to the plate-glass with a good deal of cement, it will be found convenient, after the section has been ground quite thin, to remove most of this cement from around its edges. If this is not done, it may delay the final work of grinding down. A good rock section should be made so thin that all the transparent and translucent component minerals can be examined and studied. Well made rock sections average in thickness from 0.03 to 0.05 _mm._ The last part of the final grinding and polishing can be done by the hand on a ground glass plate, using the finest emery dust or rouge and water. Flour emery may also be used entirely for grinding, and the finest emery dust for polishing. The finer emery makes better work, although it takes more time, and is much safer for porous or brittle sections, as the danger of tearing away pieces of the section is avoided. In the case of a porous or decomposed rock it is well before grinding to boil the section in Canada balsam or other equivalent material so as to fill all the pores and interstices, thus making the section more compact and less liable to chip away. Sometimes it may even be best before grinding to mount the section on its permanent glass slide, so that when it has been ground thin enough the cover can be put on without running any risk in transferring the section. The only objection is that the glass slide may be a little scratched by the emery. Mounting.