Now, the combination "flat" (diagonal and supporting pieces) and the flat surface of the second disk must be ground together with one-quarter strokes, first with No. 600 alundum, and then with No. 305 emery, after which you are ready for polishing.

The positions of the disks should be inverted for each successive charge, using eight charges of No. 600, and six of No. 305. When the fine grinding has been completed, both surfaces should test perfectly flat with a good straightedge laid across any diameter.

The Lap. The paraffin-cemented combination cannot be made use of in making the lap, as obviously no heat must be allowed to penetrate it. A channeled lap can be made on the second disk, but a sufficiently large piece of plate glass will have to be used for the preliminary shaping of the surface.

For a molded lap, proceed according to the instructions in Chapter IV, making it
on the piece of scrap plate glass, which in this case must be substituted for the "mirror," that is, for the combination flat. For flat work, the facets of the molded lap should be uniformly spaced throughout, so another rubber mat will have to be made.

If it is thought too much work to make separate mats for mirror and flat, then one of regularly spaced holes, having a 1/16" tangential separation, should be made to serve for both operations. This will work about as well on the primary mirror as the one with graduated spacing, and it has the advantage of symmetry.

After coating the lap with beeswax, soften it in hot water for a few minutes. The scrap piece of plate glass is once more called into use. Paint it with a thick mixture of the polishing compound, and when the lap is removed from the hot water, work the plate glass vigorously on it, pressing down the uneven wax coating, and
charging it well with the polishing agent.

Again soften the lap in hot water; then press into contact with the combination flat,
first spreading a piece of onion sacking over the lap. Let stand for about 15 minutes, using about 10 to 15 pounds weight in pressing; use the board that was employed in mirror pressing.

next Polishing and Testing