120 BELL SYSTEM TECHNICAL JOURNAL 



crate masses of panicles merely float through the field of vision and, 

 compared with the individual particles, appear exceedingly sluggish. 

 Fig. 20 gives a general view of the Zeiss ultra-microscope as originally 

 devised by Siedentopf and Zsigmondy. The equipment has been 

 superseded to some extent by the later Siedentopf cardioid ultra- 

 microscope. The latter is a very powerful light-concentrating device 



I-ig. iO— The Slit ultra-microscoiic for transparent solid or liquid specimens. 



and U)v this reason it is |)riniarily .ulajjlcil for iIk' exaniinatioii of fine 

 colloidal solutions and dilute precipitates as well as for the observa- 

 tion of micro-chemical and photo reactions. For transparent solids 

 and for the precursory examination of liquids and for rapidly passing 

 in review several fluids in succession, the original arrangement retains 

 marked advantages. The cardioid ultra-microscoi^e will be described 

 more fully later on. 



Fig. 21 shows diagrammaticali>- the path of the rays within the 

 preparation in the presence of ultra-microscopic particles and will 

 serve to make dearer what is to follow. In the original form of ultra- 

 microscope (Fig. 20) the horizontal incident rays which go to furnish 

 the illumination do not enter the microscope, the latter being set up 

 vertically and hence the background appears dark. The only rays 

 to enter the objective of the viewing microscope are the diffracted 

 rays which come within the aperture of the objective. 



At one end of the base board is an automatic arc lamp mounted 

 on slide rails so that it may be brought in line with either of two 

 illuminating trains mounted on optical benches. 



