146 SPECIAL PHYSIOLOGY 



as with the Fleischl instrument. Do not allow the numbers to show. 

 Begin the comparison with a colored stripe much lighter than the 

 blood specimen and move up one stripe at a time until the colors 

 appear the same; then make a reading. Next begin with a stripe of 

 a darker color than the blood and compare colors in the opposite 

 direction until they appear the same, and then make a second read- 

 ing. The correct result will be between these two readings, and 

 usually the two readings will be 10 or more per cent, apart. The 

 test is made by reflected daylight. It is well to have a good, bright 

 light, although direct sunlight is not good. 



Precautions. The amount of blood that the filter paper will 

 absorb is quite constant, and yet the amount that can be put on to 

 make the paper red is variable. If you take long enough and the 

 blood does not clot quickly, a very small amount of blood will sat- 

 urate the paper. It should be saturated quickly before the glistening 

 effect is gone, and then the amount is quite constant. This is one oi 

 the greatest errors to be avoided, and necessitates strict and accurate 

 attention to details. The error made with reading is the same as with 

 the other color tests, except as the specimen changes color. A spot 

 of blood 1 cm. in diameter is not large enough to compare with the 

 large red stripe of the chart. Colored stripes of paper of equally large 

 size can be more accurately compared than when one is very small; 

 the eye is overpowered by the larger amount of color. Many shades 

 of color before the eye at one time are confusing; so it is important 

 that all colored stripes should be covered except the one being used. 

 The above common errors are partly responsible for the disrepute in 

 which the method is held in the minds of some observers. The 

 errors are of such a nature that the accuracy of the test depends 

 almost entirely upon the operator. The technique is easily and 

 quickly performed, but the beginner should repeat the test until he 

 can obtain the same result a number of times with the same blood. 



The filter paper containing blood is wet and will destroy the colored 

 stripes if it touches them. 



E. Estimation of Percentage of Haemoglobin of the Blood by 

 Finding the Specific Gravity. 



The specific gravity of the blood can be obtained direct from a quan- 

 tity of blood as with other solutions. This is not necessary, because 

 when a drop of any fluid is put in another fluid of the same specific 

 gravity that the drop does not mix with, it will go to the center of the 

 latter fluid and remain there. If it is lighter it will come nearer the 

 surface, and if it is heavier it will sink. There are a number of solu- 

 tions that might be used. One of the most accurate is sodium sul- 

 phate in solution, placed in different cylinders in different strengths. 



The specific gravity of the blood, except in some cases, as in 



