124 Transactions of the Society. 



bacillus. With his knowledge thus acquired the plague officer goes 

 forth to war against the disease with a confidence of success in his 

 struggle that he has never known before, and in areas in which 

 cleanliness and freedom from rats, mice and fleas can be ensured, 

 we reed never fear the ravages of a plague, which, in the epidemic 

 described in Boccaccio's " Decameron " (1348), is said to have 

 destroyed one quarter of the population of the whole world, whilst 

 it is calculated that near home, in East Anglia, at least half the 

 population succumbed to this disease. 



Without the Microscope Pfeiffer's phenomenon, the solution of 

 bacteria in the fluids of living immunized animals, could never 

 have been observed, and our acquisition of knowledge concerning 

 protection against disease would have been sadly cramped. Without 

 the Microscope there could have been no Durham-Bordet or Widal 

 reaction, and diagnosis in cases of typhoid fever could never have 

 gained its present accuracy. Without the Microscope we could 

 not have put our finger on the typhoid carrier — probably the cause 

 of all endemic typhoid and of many epidemic outbreaks — and 

 without its aid Wright could have devised no opsonic index, no 

 knowledge of phagocytosis could have been acquired, and we 

 should lack all that these mean in the study and treatment of 

 microbic disease. 



It would take me too long to give a history of the twentieth cen- 

 tury development of protozoology, now the most important branch of 

 microscopic parasitology ; but as this subject is one that must occupy 

 a great place in the history of medicine, it may be well again to 

 insist that it could not have come into existence without the aid of 

 the modern Microscope, and the Eoyal Microscopical Society has 

 cause to congratulate itself that its members have continued to 

 work and to improve the Microscope even beyond what appeared 

 to be necessary to meet the immediate requirements of the botanist 

 and the zoologist. Malaria, which has probably accounted for more 

 illnesses and deaths than the bubonic plague and pneumonia 

 together, is gradually being driven from its fastnesses. Time fails 

 me to recount the stages by which a knowledge of the life-history of 

 the infective agents in the different types of malarial disease has 

 been acquired. Minute blood parasites were first found in the 

 human blood, in the red blood corpuscle and in the plasma ; then 

 their development in shed blood was careiully studied. For long no 

 solution of the problems: How do these parasites pass from man 

 to man ? anil why is malaria specially associated with low-lying 

 swampy ground ? was obtained. Then similar parasites were found 

 in birds, and Manson urged Boss to study the mosquito, infesting the 

 regions inhabited by these birds, and to see whether he could find 

 any stage of the bird parasite in the stomach or glands of the 

 mosquito, and Boss did succeed in tracing certain phases of develop- 

 ment of these parasites in the stomach wall of this mosquito. The 



