Going further down the tube past a diaphragm or two one comes to the clamping screws _F_. These serve to hold the instrument to its mounting. They may be set in separate bases screwed in place on the inside of the tube, or may be set in the two ends of a lengthwise strap thus secured. They are placed at the balance point as nearly as may be, generally nearer the eye end than the objective. Then, after one or more diaphragms, comes the guide ring _G_, which steadies the main draw tube _H_, and the rack _I_ by which it is moved for the focussing in turning the milled head of the pinion _J_. The end ring _K_ of the main tube furnishes the other bearing of _H_, and both _G_ and _K_ are commonly recessed for accurately fitted cloth lining rings _L_, _L_, to give the draw tube the necessary smoothness of motion. For the same reason _I_ and _J_ have to be cut and fitted with the utmost exactness so as to work evenly and without backlash. _H_ is fitted at its outer end with a slide ring and tube _M_, generally again cloth lined to steady the sliding eyepiece tube _N_. This is terminated by the spring collar _O_, in which fits the eyepiece _P_, generally of the two lens form; and finally comes the eyepiece cap _Q_ set at the proper distance from the eye lens and with an aperture of carefully determined size. One thus gets pretty well down in the alphabet without going much into the smaller details of construction. Both objective mount and ocular are somewhat complex in fact, and the former is almost always made adjustable in instruments of above 3 or 4 inches aperture, as shown in Fig. 28, the form used by Cooke, the famous maker of York, England. Unless the optical axis of the objective is true with the tube bad images result. [Illustration: FIG. 28.—Adjustable Cell for Objective.] To the upper end of the tube is fitted a flanged counter-cell _c_, to an outward flange _f_, tapped for 3 close pairs of adjusting screws as _s__{1}, _s__{11} spaced at 120° apart. The objective cell itself, _b_, is recessed for the objective which is held in place by an interior or exterior ring _d_. The two lenses of the achromatic objective are usually very slightly separated by spacers, either tiny bits of tinfoil 120° apart, or a very thin ring with its upper edge cut down save at 3 points. This precaution is to insure that the lenses are quite uniformly supported instead of touching at uncertain points, and quite usually the pair as a whole rests below on three corresponding spacers. Of each pair of adjusting screws one as 1 in the pair _s__{11} is threaded to push the counter cell out, the adjacent one, 2, to pull it in, so that when adjustment is made the objective is firmly held. Of the lenses that form the objective, the concave flint is commonly at the rear and the convex crown in front.