Some forms of filter candle are made with the open end contracted into a delivery nozzle, which is glazed. In this case the apparatus is fitted up in a slightly different manner; the fluid to be filtered is contained in an open cylinder into which the candle is plunged, while its delivery nozzle is connected with the filter flask by means of a piece of flexible pressure tubing (previously sterilised by boiling), as in figure 39. IV. THE MICROSCOPE. The essentials of a microscope for bacteriological work may be briefly summed up as follows: [Illustration: FIG. 40.--Microscope stand.] The instrument, of the monocular type, must be of good workmanship and well finished, rigid, firm, and free from vibration, not only when upright, but also when inclined to an angle or in the horizontal position. The various joints and movements must work smoothly and precisely, equally free from the defects of "loss of time" and "slipping." All screws, etc., should conform to the Royal Microscopical Society's standard. It must also be provided with good lenses and a sufficiently large stage. The details of its component parts, to which attention must be specially directed, are as follows: [Illustration: FIG. 41.--Foot, three types.] ~1. The Base or Foot~ (Fig. 40, a).--Two elementary forms--the tripod (Fig. 41, a) and the vertical column set into a plate known as the "horse-shoe" (Fig. 41, b)--serve as the patterns for countless modifications in shape and size of this portion of the stand. The chief desiderata--stability and ease of manipulation--are attained in the first by means of the "spread" of the three feet, which are usually shod with cork; in the second, by the dead weight of the foot-plate. The tripod is mechanically the more correct form, and for practical use is much to be preferred. Its chief rival, the Jackson foot (Fig. 41, c), is based upon the same principle, and on the score of appearance has much to recommend it. ~2.~ The ~body tube~ (Fig. 40, b) may be either that known as the "long" or "English" (length 250 mm.), or the "short" or "Continental" (length 160 mm.). Neither length appears to possess any material advantage over the other, but it is absolutely necessary to secure objectives which have been manufactured for the particular tube length chosen. In the high-class microscope of the present day the body tube is usually shorter than the Continental, but is provided with a draw tube which, when fully extended, gives a tube length greater than the English, thus permitting the use of either form of objective. [Illustration: FIG. 42.--Coarse adjustment.] [Illustration: FIG. 43.--Fine adjustment.] For practical purposes the tube length = distance from the end of the nosepiece to the eyeglass of the ocular. This is the measurement referred to in speaking of "long" or "short" tube.