TWENTY-FIVE YEARS OF BACTERIOLOGY 143 



rays of light will be intercepted and the particles illumi- 

 nated. They in turn, and according to their size, will 

 appear as bright objects, or when very small, give a dif- 

 fuse luminosity to the field. The phenomenon is similar 

 to the one described by Tyndall, in which a beam of light 

 passed through a dark space containing suspended par- 

 ticles causes them to become visible. When the suspended 

 matter consists not only of dispersed particles, but of mi- 

 croorganisms, these also become luminous, and when, as 

 with the spirochete, they exhibit a wavy structure and in- 

 dependent motion, they at once arrest attention. To-day 

 the dark-field microscope is found in every well-equipped 

 clinic, and it has aided in adding many new species to the 

 already considerable number of microbes known to be dis- 

 ease-producing. 



The latest significant addition to this field is the Lep- 

 tospira icteroides, or the jaundice-producing spiral, which 

 Noguchi has recently detected in the blood and internal 

 organs of cases of yellow fever. His extensive investi- 

 gations carried on in Ecuador, Mexico, and Peru, as well 

 as at the Rockefeller Institute, have rendered it highly 

 probable that this spirochete is the microbic incitant of 

 that severe epidemic disease. 



Yellow fever, as you know, is an insect-borne disease 

 and arises from the insertion into the blood of man of a 

 virus carried by a particular mosquito Stegomyia calo- 

 pus. After the mosquito transporting the virus has bitten 

 a healthy person, an interval of about five days elapses 

 before his blood becomes infective, and the infectiousness 

 endures about three days longer. During the latter pe- 

 riod the blood serum can be passed through the finest- 

 grained porcelain filters without losing its infectivity. On 

 the other hand, a normal mosquito which has bitten a 

 yellow fever patient, does not become capable of infect- 

 ing other human beings until after about twelve days. 

 Hence the insect acts not merely passively, as a needle 

 might, as the conveyer of the virus, but it is necessary 



