542 APPENDIX 



by the centrifuge did not dissolve sheep red cells. On the addition, 

 however, of the substance sensibilatrice it dissolved them completely. 



The addition of red cells in the experiments was always in the form 

 of a 5 per cent, mixture or suspension in 0.85 per cent. i. e., isotonic, 

 salt solution. 



The significance of the last of the above-cited experiments is, accord- 

 ing to Ehrlich, at once apparent. It is that the substance sensibilatrice 

 possesses one combining group with an intense affinity (active even at 

 C.) for the red cell, and a second group possessing a weaker affinity 

 (one requiring a higher temperature) for the alexin. 



So-called Ultramicroscopic Examinations. 



The apparatus constructed by Siedentopf and Zsigmondy 1 makes 

 visible in colloidal solutions very minute particles, which heretofore 

 could not be seen even with the highest magnifications. Particles as 

 small as a few microns are thus rendered visible. 



FIG. 164 



Virulent diphtheria bacilli. Cultures two days old. Unstained. X 2400. (After Siebert.) 



This increased power in microscopic analysis is made possible by focal 

 lateral illumination of the object to be examined. The greater the 

 difference between the refractive index of the particles colloidally dis- 

 solved and the fluid which surrounds them, the brighter will be the 

 appearance of the particles, and, therefore, the more readily visible. 

 The illumination for the purpose is most intense, and furnished by an 

 electric arc lamp. 



The microscopic field, as will be seen by the photogram herewith, 

 is dark; the objects which refract the light show as brightly illuminated, 



1 Annalen der Physik, 4te Folge, Band 10. 



