STOPPERS, REMOVING FIXED Prevention being better than cure, both necks and stoppers of glass bottles should be lubricated with tallow, vaseline, etc. The various methods of removing stoppers have been summarised as follows :—" (1) Press the stopper (longways) in one direction with the thumb (grasping the bottle with the fingers of the same hand) ; now with the other hand hold a chisel or file by the iron part and tap the opposite end of the stopper in the contrary direction to which you are press ing it. (z) Place the stopper in or under a damp —as that of a carpenter's bench—first wrapping something soft round it, as a piece of leather or wool. Considerable leverage can thus be obtained, but it must not be used to too great an extent or the stopper will snap off. (3) A substitute for the wooden clamp is the use of a key whose handle is just large enough to go over the wrapped-up stopper. These three may be termed the mechanical methods. The use of heat is also very effective in expanding the neck of the bottle. For this purpose we may (4) invert the bottle and dip stopper and neck into a saucepan of hot water. A string should be tied round the stopper or it may drop out and the contents of the bottle be lost. (5) Rotate the neck of the
bottle rapidly in the flame of a Bunsen burner.
This is a very effective method, but requires care. (6) Take one turn of a stout piece of string round the neck, and then by means of the two ends saw the string rapidly backwards and forwards. The heat produced by friction will cause the neck to expand." Stoppers may become fixed owing to the evaporation of their contents, the solid matter crystallising between the neck and the stopper. The remedy is to pour some of the solvent used to make the solution round the stopper and renew as may be necessary, giving it time to work its way between stopper and neck.
The use of a smaller stop to decrease the aperture of a lens with the object of improving the depth of focus or definition, of reducing spherical aberration to a negligible quantity, etc. (See " Diaphragms.") Stopping-down directly affects the duration of exposure, as explained under the heading " Exposure Tables." (See " Blocking-out.")