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 (1913),975. King, G.: J.Soc. Ch.Ind.38(1919),4. 
