i6 



BACTERIOLOGY 



objective lens system, and made possible the construction of 

 achromatic objectives, perhaps the most important advance ever 

 made in the construction of the microscope. Abbe (about 1880) 

 introduced his substage condenser which made possible the in- 

 tense illumination of the microscopic field. In collaboration with 

 Zeiss, Abbe (1886) devised an objective lens system with more 

 perfect chromatic correction than had been previously attained. 

 These objectives are constructed of several different kinds of 

 glass and have in addition one lens composed of rmorite. Sieden- 

 topf and Zsigmondi (1903) devised a method of illuminating the 

 microscopic preparation by horizontal beams and so brought to 



FIG. i. The formation of an image by means of a simple pin-point aperture. 



(After A, E. Wright.} 



view exceedingly minute refractive particles as luminous points 

 on a dark field. The various dark-field condensers introduced 

 in recent years (1906) utilize similar principles, the object being 

 illuminated by oblique light. Recently, Gordon has devised 

 the tandem microscope, an instrument which has demonstrated 

 the possibility of achieving greater microscopic resolution than 

 has previously been attained and even suggests that there is no 

 necessarily final limit to the degree of magnification at which 

 satisfactory definition and resolution may be achieved. 



Principle of the Microscope. The formation of an image by 

 means of a simple pin-point aperture is illustrated in Fig. i. It 

 will be noted that the magnification achieved is the quotient of 



