NORMAL H^EMATOLOGY 141 



the handle of the capillary and stir the one that contains the blood 

 so as to make the mixture complete. Now carefully slide the thick 

 cover-glass over the compartments and gradually fill each cell with 

 water as the cover-glass is put on until there is no air left in either 

 cell. Exclude daylight by use of a dark room or a cupboard, and 

 adjust the reflector so that the artificial yellow light is thrown up 

 through the diluted blood and water from the side of the instrument, 

 thus placing both cells in same relation to the reflector and the light. 

 While making the test always shade the eyes from the light by placing 

 some thick paper or a pasteboard tube, that reaches from the instru- 

 ment to the forehead, before the eyes. It is better to use only one 

 eye at a time, and look only for a few seconds at each time, giving 

 the eye a rest and a chance to regain the ability to distinguish tints. 

 Stand at one side of the instrument or turn the instrument so as 

 to face the light and to bring the two cells into similar relations 

 with the eye. Begin with a glass of a lighter color than the blood, 

 and move the colored-glass slide by quick turns about one-fourth 

 of an inch each time until the color or tint of the diluted blood appears 

 to be the same as that of the colored slide; then make the reading. 

 Next turn the colored glass on until it is darker than the diluted 

 blood and do the same as before, except in the opposite direction, 

 turning the slide until the color of the glass and blood are the same, 

 and then make the reading. Usually the first reading will be too 

 low and the second too high. The difference will usually be about 

 10 per cent. The correct result will be between these two readings, 

 which can now be obtained by carefully moving the glass back and 

 forth or by taking the middle point between the two readings. It is 

 almost impossible to make the reading accurately and honestly unless 

 great care is taken, and the writer has found the method given to 

 produce the best results by far. This method should be practised again 

 and again and done with care. A hasty reading is rarely correct. 

 Repeat the whole test until you can obtain the same result each 

 time with the same individual's blood. 



Precautions. If the capillary tube is not perfectly clean it will 

 not take blood by capillary attraction. While cleaning the tube 

 always test it by touching a drop of water, when it should fill imme- 

 diately. This will save time and ensure quick work. The amount 

 of blood taken is so small and this is diluted so much that the least 

 error is multiplied many times. We can expect accurate results 

 only when every known chance of error is safely guarded. If the 

 capillary has moisture or foreign matter in it, the tube will not hold 

 the right amount of blood and the result will be too small. The 

 blood must be obtained and mixed in the metallic cup with the 

 water very quickly, or it will clot and stick in the capillary, or if it 

 does leave it it may remain as a clotted thread of blood in the 

 bottom of the cell. It takes a little practice to learn to wave 



