The focusing screw and standard should follow closely that of the sample, except that the milled focusing disk should have a face as nearly one-half inch wide as possible and the milling should be sharper. In addition to the diaphragm upon which the automatic lens is mounted, there shall be two diaphragms in each tube, so situated and so proportioned as to cut off all stray light and all internal reflections. The crossbar supporting the draw tubes should be shaped and engraved exactly as found in the model. (_d_) The lenses must be entirely free from mechanical defects, such as specks, air bubbles, etc.; must be free from interior strain, and must be ground from the best obtainable glass for the purpose, selected for general transparency, as colorless as possible, perfectly ground and polished, and accurately centered. _The object lenses_ shall be composite, achromatic, and well corrected for spherical aberration, with a clear aperture of at least 1½ inches, and not exceeding 1-5/8 inches. Bidders will state the number and shape of the pieces used to make up this lens. _The compound lenses_ may be either cemented together with Canada balsam, or left uncemented, as the maker may deem best for durability and optical performance, but if left uncemented the components shall have a permanent mark to indicate their proper positions in the cell. _The eyepieces_ shall consist of a single double concave lens having a clear aperture of not less than three-eighths of an inch and not more than one-half of an inch. (_e_) The sling cord attached to eyes in the frame by means of brass snaps with black burned finish shall be round and braided from four strands of pliable tan leather, and shall have a diameter of at least one-eighth of an inch and not over one-sixth of an inch. (_f_) _The case and strap_ must be exactly like sample, and of No. 1 stock. Care must be taken to put in only compasses that are in perfect condition. The strap buckle must be of brass. The glass, when closed, must not exceed 4 inches in length, and the glass, case, cord, and strap, complete, must not exceed 25 ounces in weight. (_g_) The frame shall be constructed with jointed bars for interpupillary adjustment. 5. _Service field glass, type B._--(_a_) The requirements of part 4, service field glass, type A, of this specification, shall be followed in the design and construction of the type B glass in so far as applicable. (_b_) Power: The lower power shall be approximately 4½ and the high 6½ diameters. (_c_) Object lenses: These shall have a clear aperture of at least 1¾ inches diameter. (_d_) Case: Case and carrying strap shall be furnished as required in part 4 of this specification. (_e_) This glass shall be constructed with jointed bars for interpupillary adjustment. (_f_) The sunshade, when drawn out, shall project not less than three-eighths of an inch and not more than 1 inch beyond the edge of the cell.