ULTRAMICROSCOPY 



65 



the disadvantage that it cannot be used with objectives of 

 immersion ; for total reflection will no longer necessarily be 

 produced at the upper surface of the coverslip if, instead of 

 air, there is some other fluid. 



In the two arrangements described above, the illuminating 

 rays and those employed for observation have their axes at 

 right angles. Though this condition is necessary when very 

 small particles are to be observed, yet, for somewhat larger 



Fig. 4. 



Fig. 4A. 



objects, such as cells, fibres and bacteria, a co-axial arrangement 

 can be used. " The arrangement can be made nearly identical 

 with that employed in ordinary microscopic vision, so that 

 a transition from ordinary to ultramicroscopic observation and 

 vice versa is at all times easily accomplished." 



Figs. 4, 4A, 4B and 5, reproduced with M. Zeiss and Co.'s 

 permission, illustrate two methods now frequently used in the 

 examination of fibres and bacteria. In these figures the illumi- 

 nating rays are shown by continuous lines, the diffracted rays 

 by dotted lines. 



5 



