VACCINE-THERAPY 67 



heat to fix these films is scarcely practicable, unless in 

 highly skilled hands. As fixing agents we therefore use 

 formalin or corrosive sublimate. The latter is probably the 

 most useful, speaking generally. 



The slide is placed in the slide-rack containing a satu- 

 rated aqueous solution of hyd. perchlor. for half to one 

 minute. It is then washed under the tap, air dried, and 

 stained with carbol-thionin, but in the case of tubercle by 

 carbol-fuchsin (hot). 



The best film of each should now be examined by a low 

 power, to ascertain which is the best, and then by a -j^- oil- 

 immersion lens, and many of the polynuclear leucocytes 

 will be found to contain bacteria. 



A pencil and a piece of paper should then be placed 

 at the side of the microscope, and a count of the leucocytes 

 containing bacteria made up to fifty, first, from the slide 

 with the control serum, secondly, from the slide with the 

 patient's serum, the ratio between the two giving the 

 opsonic index. 



Thus, if the control serum film showed that the 50 

 leucocytes had ingested 150 bacteria, and the patients' 

 serum film showed that the 50 leucocytes had ingested 

 80 bacteria, the ratio 80^-150=0*53 gives the opsonic 

 power of the patient; or, assuming the control animal's 

 opsonic power to be normal, the patient's would be nearly 

 50 per cent, below normal. 



In such a condition an animal is a bad subject to resist 

 bacterial invasion. It has been shown that a patient may 

 have a very high opsonic index against a bacterial infection 

 at a certain stage of the disease, and yet die. This state is- 

 usually explained by reason of the fact that secondary in- 

 fection has taken place, and secondary bacteria are the 

 cause of dissolution. 



A very low opsonic power suggests either that the animal 

 is suffering from the direct effects of a specific infection, or 

 its resisting powers are so low as to make it susceptible to 

 infection should opportunities occur. This explains largely 



