76h DARK-GROUND ILLUMINATION WITH HIGH POWERS [Cn. U 



justed for twilight vision and can most easily make out the finest 

 details. 



132a. Example illustrating the method of procedure in dark-field micro- 

 scopy. Perfectly fresh blood is one of the best objects to study by this method. 

 As pointed out by Dr. Edmunds nearly 50 years ago, blood with the dark- field 

 illumination appears like a new object so many things are seen with the greatest 

 distinctness that are wholly invisible or merely glimpsed when examined by the 

 bright- field method. 



(1) Carefully cleaned slides and cover-glasses of the right thickness are placed 

 where they can be easily grasped. 



(2) For obtaining the fresh blood the part to be punctured should be cleaned 

 well with 95 % alcohol and then with a sterilized needle or Dr. Moore's Haemos- 

 past, the puncture is made. The drop of blood exuding can be quickly touched 

 by a cover-glass, and the cover put on the center of one of the prepared slides. 

 If a. small amount adheres to the cover, it will spread out in a very thin layer when 



corpuscles 



after the preparation is on the slide. 



If all the preparations are quite red, after a few minutes, one can be made 

 thinner by pressing firmly on the cover by the ball of the thumb covered with gauze 

 or lens paper. The gauze or paper absorbs the blood which runs out at the edge 

 of the cover. In order to prevent evaporation and to help anchor the cover- 



tlass so that it will not move by the pull of the viscid homogeneous immersion 

 uid, it is advisable to seal the cover by painting a ring of liquid vaseline (petro- 

 leum oil) or castor oil around the edge of the cover. One of the thick preparations 

 should not be sealed, but kept for irrigation with normal salt to show especially 

 the fibrin network. When ready to study the blood, put a large drop, or two large 

 drops, of homogeneous liquid on the underside of the slide directly opposite the 

 specimen, and place the slide on the stage of the microscope so that the immer- 

 sion liquid will come over the face of the condenser. Then a drop of immersion 

 liquid is put on the cover-glass and the objective run down into it. If the light- 

 ing is secured as explained above one soon learns to focus on the specimen. In 

 general, the field all looks bright just before the objective gets down to the level 

 for seeing the specimen. 



(a) The erythrocytes will appear like dark discs with bright rims owing to the 

 convex borders. 



(b) The leucocytes appear as real white corpuscles owing to the granules within 

 them which turn the light into the microscope. If the room is moderately warm 

 20 C. or more the leucocytes, some of them, will undergo the amoeboid 

 movement, and the picture they present will be a revelation to those who never 

 saw it or only with the bright-field microscope. From the clearness with which 

 everything can be seen the minutest change can be followed, and also the most 

 delicate pseudopod detected. Another striking feature will be noticed in the 

 moving ones, that is, the vigorous Brownian movement of the granules in the 

 part of the leucocyte with the amoeboid movement. In those showing no amoe- 

 boid movement there is usually no sign of the Brownian movement of the granules; 

 also if a part of the leucocyte is not undergoing amoeboid movement the particles 

 in it are usually motionless. 



(c) The fibrin network will be seen like a delicate cob-web between the cor- 

 puscles. In different parts of the specimen one can find all the appearances of the 

 fibrin shown in text books on the blood. 



