No amount of simple blackening will answer the purpose, for even dead black paint such as opticians use reflects at very oblique incidence quite 10 to 20 per cent of the beam. The importance of both diaphragms and thorough blackening has been realized for at least a century and a half, and one can hardly lay too much stress upon the matter.
The diaphragms should be so proportioned that, when looking up the tube from the edge of an aperture of just the size and position of the biggest lens in the largest eyepiece, no part of the edge of the objective is cut off, and no part of the side of the tube is visible beyond the nearest diaphragm.
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.
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 s1, s11 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 s11 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.