ULTRAMICROSCOPES 65 



crater of the electric arc, and consequently unequal illumina- 

 tion of the slit results or there is a failure (due to a nickering arc) 

 of the spot of light to remain centered upon the slit. Holding 

 the black screen against the lens C 2 , on the side toward the slit, 

 from time to time, will show when the arc needs adjusting, 

 since there should appear a spot of light of uniform intensity 

 and in the proper position to fall concentric with the optic axes 

 of Ci, C 2 , C 8 . 



When dealing with exceedingly fine colloidal particles it is 

 often an advantage to cut off the lower half of the beam by 

 means of a screen mounted upon a saddle stand and placed 

 between S and C 2 , the upper horizontal edge of the screen being 

 raised so as to cut off the lower half of the beam of light. Ap- 

 proximately as good results may be obtained more easily by laying 

 against the end of the tube T a small rectangular piece of black 

 hard rubber or blackened brass d, as shown in the diagram. 



Reflecting Condenser Ultramicroscopes consist of highly 

 perfected dark-ground illuminators applied to ordinary micro- 

 scopes provided with special objectives of low numerical aperture. 

 In the special condensers used, the light rays are reflected from 

 two spherical surfaces. The illuminating rays therefore enter 

 the preparations with obliquities greater than in ordinary dark- 

 ground illuminators and are brought to a correct focus. 1 



By employing objectives of low numerical aperture (about 

 0.85) we have rays including only a low range of apertures taking 

 part in the formation of images, although the illuminating rays 

 include a range of high aperture, i.i to 1.35. There is thus 

 obtained the greatest brilliancy of image upon the darkest of 

 backgrounds. 



Although many different ultracondensers are obtainable, 

 space forbids a consideration of more than two types: the car- 

 dioid condenser of Siedentopf as made by Zeiss, and the ultra- 

 condenser of Jentzsch as made by Leitz. 



1 In the ordinary paraboloid condensers, when properly constructed, the light 

 rays are also brought to a focus, but the focal length varies from zone to zone, 

 hence we have an overlapping of images at the center. (Zeiss, "Mikro" Circular 

 306, p. 8.) 



