i6 
The standards fade in the tropics and must be 
returned for graduation. 
Procedure .—I. Place a little water in the gradu¬ 
ated tube. 
2. Fill the pipette carefully, holding it almost 
horizontal, and wipe off any blood on the outside. 
3. Blow out into the tube. Now wash out the 
pipette thoroughly by sucking up and blowing out 
successive lots of water from the rubber pipette supplied. 
4. Add water by successive amounts of five 
scale divisions and stir thoroughly with a piece of wire 
or probe. 
Sahli .—Is constructed on the same principle as the 
Gowers, except that in this case the standard is one of 
acid haematin. The blood to be examined must 
therefore be treated with about ten times its volume of 
one-third per cent, hydrochloric acid; then fill up with 
water. 
It is a very accurate instrument. Gently shake 
the standard when a sediment occurs. 
Fleischl-Miescher .—Fully described in the book 
accompanying each instrument. It is very accurate 
but expensive. 
For choice use a Talqvist and Sahli. 
Colour Index 
When the haemoglobin is 100 per cent, and the 
red cells five million (i.e., with the ratio 
the colour index is said to be 1. 
If this ratio were equal to 1 the colour index 
would then be 50,000. So that to find the colour 
index, after counting the red cells and estimating the 
