ULTRAMICROSCOPES 123 



The Abbe condenser is removed or swung aside. The plane 

 mirror is then turned so as to reflect a beam of parallel rays into 

 the device. This beam must be of such diameter as to completely 

 fill the aperture of the condenser. A powerful source of light is 

 essential, preferably an arc lamp or concentrated filament Mazda 

 bulb. The mirror is tipped until the bright spot of light appears 

 at the center of the cell. Since in this case we are examining the 

 path of the rays as in the slit ultramicroscope and these rays 

 enter from all sides and meet at the center, it is unnecessary to 

 exactly center the condenser. 



Special objectives of great penetrating power are necessary, 

 corrected for the thickness of the quartz plate Q and whose 

 mountings are of sufficiently small diameter to permit their 

 entrance into the well in the cover to a depth such that the focal 

 point will lie within the path of the rays. High magnifications 

 must be obtained by employing high power eyepieces. It 

 follows that there is always an illuminated plane lying below 

 the focal plane of an objective and a perfectly black background 

 is unobtainable. In order to obtain sharper contrasts, a dia- 

 phragm can be placed just above the mirror, either cutting off 

 one side of the beam of light or having an opening slightly eccen- 

 tric to that of the annular opening in the ultramicroscope. 



Great care must be exercised in cleaning the cell walls and the 

 quartz plate. 



For coarse colloids and for suspended matter in vapors and 

 water the author has found this device of great convenience and 

 a time and labor saver; but for very fine suspensions the results 

 are not so good. 



The Immersion Ultramicroscope. — In this instrument de- 

 vised by Zsigmondy * we have the most improved type of micro- 

 scope for the study of ultramicroscopic particles yet devised; 

 through the employment of immersion objectives of high numer- 

 ical aperture for both illumination and observations, much more 

 brilliant and sharper diffraction disks are obtainable. Thus the 

 existence may be demonstrated of particles even smaller than 

 those rendered visible by ultramicroscopes of the cardioid type. 



'Zsigmondy: Physik. Zeit., 14 (iqi3),q7s. King,G.: J.Soc. Ch.Ind. 38(1919), 4. 



