528 ILLUSTRATIVE EXPERIMENTS 



B. Polarisation of the. Tyndall Cone. Fill the cell with a mastic sol. Cut 

 down the light from the lantern to the minimum, which will still give a well- 

 defined cone. Examine the cone with a Nicol prism [q.v.) held at right angles 

 to the beam of light and rotate the prism. The cone of light will become very 

 dim twice in every complete revolution of the prism. That is, the light is 

 partially plane polarised. 



Another similar cell should be ready to be substituted for the one containing 

 the mastic sol, or should be placed in series with it and nearer to the source 

 of light. Fill the second cell with a fluorescent solution such as that of 

 quinine bisulphate or of fluorescein or of eosin. Fluorescent solutions pro- 

 duce a cone, but no polarisation of the light. 



28. Ultra-microscope. 



The ■■ slit "" ultramicroscope and the cardioid condenser require special 

 lighting, but the coarser ultramicroscopic particles can be seen by the use 

 of a dark-ground condenser such as that shown in Fig. 15 on p. 80 (or 

 Watson's or Zeiss' " Paraboloid,"' Reichert's " Table," Jentzschs "' Con- 

 centric,"' or Watson's '" Nelson Cassegrain "). The last-named condenser 

 can be used with any oil immersion objective utilising its full aperture 

 without a funnel-stop in the objective. It is effective through the thickness 

 of an ordinary microscope slide. 



Paraboloid Condensers (Watson or Zeiss). These condensers are simply 

 substituted for the optical part of the Abbe condenser. A funnel-stop must 

 be placed in the oil immersion objective to reduce their aperture below 1-0. 

 The following points require attention. 



(i.) The ])araboloid must, in every case, have oil between it and the micro- 

 scope slide. All air bubbles must be excluded, and sufficient oil used to 

 maintain a perfect contact between slide and condenser. 



(ii.) The condenser must be accurately centred. 



(iii.) The fluid under examination should be placed on the slide in as thin 

 a layer as possible. 



(iv.) The illumination should be as brilliant as possible. Good results will 

 be obtained by the use of an Ediswan " Point-o-lite " with bull's-eye con- 

 densing lens. 



(v.) The slides and coverslips must be clean. The best method is to 

 immerse them in hot bichromate-sulphuric acid mixture for 5-10 minutes. 

 Rinse thoroughly in distilled water. Pick up the slides and slips one at a 

 time with forceps ; shake off the bulk of the water and place in dry alcohol 

 till required. When required withdraw a slide from the alcohol {using 

 forceps) and burn off the adherent alcohol in a spirit lamp. As soon as the 

 slide has cooled place it on the oil on the condenser. The slips may be treated 

 in the same way, or, if they crack readily, the alcohol may be removed by 

 evaporation near the flame. The cover slip, as soon as it has cooled, is placed 

 gently on the drop of sol to be examined. At no time should the slips or slides 

 be touched by hand or by cloth. 



A. Mastic sols, gold sols and suspensions like gamboge and India ink show 

 up well. 



B. Brownian Movement (p. 82 and Fig. 16). (1) Clean slides and cover 

 glasses in hot potassium bichromate-sulphuric acid mixture, rinse in distilled 

 water and then in two changes of alcohol. Keep till required in alcohol (see 

 Experiment 28 above). 



(2) When ready to examine a sol, withdraw a slide from alcohol (using 

 forceps) and evaporate dry over a clean flame. Cool. A large drop of 

 sol free from air bubbles is placed on the centre of the slide. The cover 



