This is an interesting problem and demonstrates a scientific principle.

For a periscope of the size shown in the drawing (Plate 21), two pieces of looking glass must be first cut 2-1/4" × 2-7/8".

Pieces A are cut 3/8" × 2" × 4-1/4"; pieces B are 3/8" × 2-3/4" × 4-1/4"; pieces C, 3/8" × 1-7/8" × 2-3/4"; pieces D, 3/8" × 2" × 17-1/4"; pieces E, 3/8" × 2-3/4" × 18-7/8"; and pieces F, 3/8" × 2-3/4" × 18-7/8".

Two grooves 1/8" deep, and of a width to receive the thickness of the glass used, should be cut at an angle of 45 degrees, where indicated on the drawing. This groove is cut in pieces A only.

All pieces should be thoroughly sanded with No. 1 sandpaper and finished with No. 0. Assemble, as shown on the drawing, using glue and 1" brads.

The final finish may be stain or paint. Whatever finish is used should be of a dark color as best suited for a periscope.