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. 1 



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 s. 



1 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. 



