i6 



BACTERIOLOGY 



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

 introduced his substage condenser which made possible the intense 

 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 fluorite. They 

 are called apochromatic objectives. Siedentopf and Zsigmondi 

 (1903) devised a method of illuminating the microscopic prepa- 

 ration by horizontal beams and so brought to view exceedingly 

 minute refractive particles as luminous points on a dark field. 

 The various dark-field condensers introduced in recent years 



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Fig. I. — The^formation of a leans of a simple pin-point aperture. 



(After A. E. Wright.) 



(1906) Utilize similar principles, the object being illuminated by 

 oblique light. Recently, Gordon has devised the tandem micro- 

 scope, 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 

 aperture-image distance divided by object-aperture distance; 



