PRESIDENTIAL ADDRESS. 27 



theory of diseases, and to increased knowledge of the 

 bacilli, which required better microscopical lenses for 

 their examination and identification than then existed. 

 This led to the manufacture of apochromatic lenses, of 

 new construction and power, which, added to other 

 improvements in the microscope, based on the principle 

 established by Lord Lister's father in the manufacture of 

 achromatic glasses in the earlier part of the century (1829), 

 seem to have apparently reached almost the highest per- 

 fection possibly required for microscopical manipulation. 



The microscope has indeed in our time "grown from 

 the comparatively rarely used possession of a few men of 

 science to be the highly finished and most important 

 companion of all investigators of Nature." 



The actual value of minute organisms as agents in the 

 very important matter of sewage purification, although 

 seemingly very much like setting a thief to catch a thief, 

 has, during recent years, been clearly demonstrated. 

 Indeed, it was not until the biologist came to the aid 

 of the chemist and engineer that a scientific method of 

 purification of sewage was brought about. By turning 

 the sewage into the sea, where circumstances render this 

 favourable, a vast increase in the number of Entomostraca 

 ensues, thus causing a corresponding increase of edible 

 fish, and so benefitting man. But it has been more 

 recently demonstrated, by the State Board of Health in 

 Massachussets, that micro-organisms are themselves of 

 the greatest value in the purification of sewage. After 

 mechanical filtration of a volume through sand or gravel 

 has taken place, sufficient air to support micro-organisms 

 is admitted, which enters the pores of the filter, and with 

 the help of the oxygen present, completely purifies organi- 

 cally impure sewage. 



Indeed, the important part played by bacteria in many 



