1 66 INTRODUCTION TO GENERAL PHYSIOLOGY 



tion is necessary. A drop of blood is obtained by pricking the 

 finger or from the heart of a frog which has been killed by chloro- 

 form. The cover-glass should have vaseline painted around its 

 edges for a millimetre or so on the lower surface before placing it 

 on the blood. This is to prevent drying of the preparation. The 

 leucocytes will be found in the spaces between the columns of red 

 corpuscles. Movements will be slow or absent in the case of the 

 human blood unless the slide is warmed to body temperature. 

 This can be done by placing the slide on a piece of sheet copper 

 which has a hole to admit the light from below. This piece of 

 copper has a long narrow part projecting a few inches beyond the 

 edge of the stage. It is warmed by means of a small gas burner, 

 the temperature not being allowed to rise at the place where the 

 slide rests on it more than just feels warm to the finger. 



Movement of Protoplasm. This should be observed in the 

 hairs found on the stamens of Tradescantia. The species, T. 

 Virginia*^ with violet flowers, is grown in most gardens. The cell 

 sap, being coloured, obscures the protoplasm somewhat, but it can 

 easily be seen in the form of a layer lining the cell wall and fila- 

 ments stretching across the cell. There are continual streaming 

 movements, and the filaments change their position from time to 

 time. Two or three hairs are picked off with forceps by seizing 

 them at their bases. Mount in water. 



There is a greenhouse species, T. discolor^ with colourless flowers 

 and variegated leaves. The cell sap being free from colour, the 

 protoplasmic movements arc more easily seen, 



Dark-Ground Illumination. The condenser used is sold in 

 various forms by dealers. Water is placed between it and the 

 bottom of the microscope slide to prevent the oblique rays being 

 reflected. The method is of especial value in observing fine 

 particles, with their Brownian movement and its cessation on 

 stimulation of protoplasm. It is very useful in observations on 

 bacteria. A brilliant source of light is required, and the small arc 

 lamp will probably be found best, using a condensing lens to make 

 the rays parallel. Large granules will appear too dazzling, but 

 the minute ones need to be well lighted. In using the method 

 with living cells, it is well to introduce a flat-sided glass vessel 

 filled with water between the lamp and the microscope in order 

 to cut off heat rays. It may also be advisable to add some 

 quinine sulphate dissolved with the aid of dilute sulphuric acid to 

 absorb ultra-violet rays. 



Brownian Movement. A cake of dry water-colour gamboge 

 is rubbed in distilled water and a drop placed on a slide. The 

 finer the particles the more vigorous the movements. They can 

 be seen quite well with the ordinary form of illumination, but much 



