ULTRAMICROSCOPES 
109 
micrometer. The slit is then rotated through 90° the diameter 
of the beam again measured and the area of its cross-section 
computed. From this value the volume of illuminated liquid 
in the field of view may be ascertained. Micrometer screws 
on the slit mechanism permit the worker to adjust the area of 
the slit opening to any convenient dimensions; the graduations 
on the micrometer circles furnish a means of recording the slit 
dimensions for future reference.^ 
The Slit Ultramicroscope consists of an ordinary compound 
microscope, a special cell of black glass with small windows at 
right angles to one another and an illumination device for pro¬ 
jecting a tiny beam of light into the cell in a line at right angles 
to the optic axis of the microscope. The tiny beam of light is 
obtained by means of small projection lenses and an adjustable 
slit. To distinguish this type of illumination from others com¬ 
monly employed in microscopy, the term “ orthogonal illumi¬ 
nation ” has been proposed. It is obvious that in this system no 
direct light can enter the objective but only such rays as are 
diffracted by the particles in suspension in the liquid contained 
in the cell. 
The form and arrangement of the component parts of the 
slit ultramicroscope naturally differ according to the optical 
firm manufacturing the instrument. One of the best known 
and most frequently used types is that shown in Fig. 51.2 This 
instrument consists of an optical bench B, at one end of which 
is placed an arc lamp R and at the other a compound microscope. 
Between the lamp and the microscope there are a series of con¬ 
densing lenses and an adjustable slit. The light rays emanating 
from the arc are collected by the spherically and chromatically 
corrected lens Ci of 80 millimeter focus, so placed as to project a 
very bright image of the crater of the arc upon the slit S. In 
ordinary use this slit has its length in a horizontal position, the 
width being controlled by the micrometer screw with graduated 
head G, while the length of the slit is regulated by the screw 5. 
^ For calculating the size of colloidal particles see: Zsigmondy-Spear, The Chem¬ 
istry of Colloids, Wiley & Sons, N. Y. 1917, page 15. 
2 Manufactured by Carl Zeiss, Jena. 
