KAHN TEST 179 
particles. If particles are present, the serum should be recentrifuged 
and retested . 
Shaking and Addition of Saline. — During the three-minute shaking 
period, it is important not merely to agitate the rack of tubes but to see 
to it that the fluids within the tubes are vigorously agitated. When 
shaken by hand, one may shake three one-minute periods with short 
rest periods. When a shaking machine is employed, its speed should be 
275 oscillations per minute, with a stroke of 1.5 inches. Shaking by 
hand should approximate this. After the tests are shaken, add 1 cc. 
saline to each tube of the first row of the rack (containing the 0.05 cc. 
amounts of antigen suspension) and 0.5 cc. saline to the remaining 
tubes. Shake sufficiently to mix the ingredients. 
Reading Results.— The results are read after the addition of salt solu- 
tion. Optimum reading conditions in each laboratory should be deter- 
mined by trial. The following points will be found helpful: (a) 
When utilizing daylight for reading the tests it is well to have but one 
source of light coming from a single window immediately in front of the 
reader. It will be found satisfactory to shade the upper and lower 
portions of the window, narrowing the source of light to a section several 
feet in height. Light from other windows near the reader should 
be dimmed by lowering the window shades. (6) When holding the 
rack in front of the exposed section of window light, the definitely 
positive anfl negative reactions are readily differentiated without lifting 
the tubes from the rack, (c) In case of weak reactions, examine each 
tube infli^'idually, lifting it for some inches above the eye level and 
slanting it until the fluid is spread into a thin layer. The precipitate 
w411 then l)ec()me readily visible. 
Those preferring magnification will find the microscopic mirror 
helpful. Place the mirror on a reading table with the concave surface 
upward. Hold the tube in a slanted position 2 to 3 inches above the 
mirror and examine the image in the mirror. Both daylight and 
artificial light may be employed. One may also utilize an ordinary hand 
lens for reading the tests. A two or three-fold magnification will be 
found satisfactory. Some workers prefer the use of a slit-light arrange- 
ment, the source of light being an electric bulb enclosed in a box which is 
])rovided with a narrow slit. 
As far as possible, workers should limit themselves to one method 
of reading. The occasional use of magnifiers by reaflers who usually 
do not resort to this method will be likely to affect the uniformity of 
their reading scale. It should be emphasized that certain highly 
magnifying agglutinoscopes show particles in serum alone and are thus 
unfit for use in this test. The magnification must be sufl^iciently low 
as to assure opalescent and clear-cut negative reactions with entire 
freedom from ^■isible particles. 
Interpretation of Results.— A definite precipitate suspended in a clear 
medium is read 4+. Proportionally weaker reactions are read 
3, 2 and 14- and doubtful, respectively. The negatives appear clear 
