200 TECHNOLOGY OF BACTERIA. 



tion of the dust deposited upon exposed surfaces, 

 or of the suspended particles collected by means 

 of an aeroscope. 



Various forms of aeroscope have been devised, 

 the object of all being to cause a current of air to 

 pass through a small aperture against a glass slide, 

 the centre of which has been smeared with glycer- 

 ine or some other viscid material, which serves to 

 retain suspended particles. In the apparatus of 

 Maddox, which was used by Cunningham in India, 

 and a modification of which is employed by Miquel, 

 a metal cone is made to face the wind by means 

 of a weather-vane to which it is attached. A small 

 aperture at the apex of the cone permits the con- 

 centrated current of air to project itself against 

 a glass slide, smeared with glycerine, which is 

 properly supported at a short distance back of this 

 orifice. In the apparatus used by Klebs and 

 Tomasi-Crudeli, in their investigations in the vicin- 

 ity of Rome, a current of air is produced by a 

 revolving " fan-wheel " moved by clock-work. The 

 writer, in his investigations in Havana in 1879, 

 and in New Orleans in 1880, used a water-aspirator, 

 by means of which a measured quantity of air was 

 caused to flow in a given time, through a small 

 aperture, and to impinge upon a glass slide smeared 

 with glycerine. Any one of these methods will 

 answer the purpose ; but the apparatus of Maddox 

 seems to be the simplest, and has yielded very 

 satisfactory results. 



Instead of collecting the suspended organisms 



