4. Dissolve twenty-five grammes of sugar in 250 grammes of water; add three grammes of tartaric acid; heat it to ebullition during ten minutes to complete the conversion of sugar; cool down, and add fifty cubic centimetres of alcohol in summer to prevent fermentation, add water to make the volume to ½ litre in winter and more in summer. _Clean well_ the surface of the glass. Take equal quantities of the four solutions: mix 1 and 2 together, and 3 and 4 also together: mix the two, pouring it at once into the vessel where the silvering is to be done. The mirror is suspended face downwards in the liquid, and the deposit begins after about three minutes, and is finished after twenty minutes. Take out the mirror, clean well with water, dry it in the air, and rub it then gently with a very fine leather. CHAPTER XI. THE “OPTICK TUBE.” Having now obtained the lenses and specula we come, in order to complete our consideration of the purely optical portion of the subject, to the question of mounting these lenses and specula in tubes and thus connecting them with the eyepieces so as to become of practical utility. We will first consider the adjustment of lenses in a tube, the combination forming a simple telescope that can be supported, in any manner desirable, by mountings we shall presently consider, according to the purpose for which it is required. The adjustment of specula will be considered as we advance further. The smaller telescopes consist of a brass tube, the object-glass, held in a brass ring, being screwed in at one end of the tube: a smaller tube sliding in and out of the other end of the large tube, generally moved by a rack and pinion motion, carries the eyepiece. In larger telescopes the mounting is similar, only somewhat more elaborate, the object-glass being carried in a brass cell, or a steel one if the dimensions are very large. This screws into the ring at the end of the tube, and this ring can be slightly tipped on either side by set screws, so that the object-glass can be brought exactly at right angles to the axis of the tube. [Illustration: FIG. 67.—Simple telescope tube, showing arrangement of object-glass and eyepiece. ]